```                   # 📋 Session Report
    λλ              
   λ⊙λ⊙             
  λλλλλλ            
 λλλλλλλλ           
    λλ              
    λλ              
    λλ              
    λλ              
    λλ              
    λλ              
    λλ              
    λλ              
```                 

**🔑 Session ID:** 0aa7134c-91a7-4ee9-b707-8b8c9378854c

---

## 📊 Statistics

### Turn Counts

| Metric | Value |
|--------|-------|
| Total Turns | 26 |
| 👤 User Turns | 13 |
| 🤖 LLM Turns | 13 |

### 🔧 Tool Calls

**Total Tool Calls:** 15

`bash_read-file      `    6 ████████████████████████████████████████████████████████████

`bash_write-file     `    6 ████████████████████████████████████████████████████████████

`bash_check-code     `    1 ██████████

`bash_grep-files     `    1 ██████████

`bash_list-files     `    1 ██████████


### 💾 Byte Usage

`Input     `    338 KiB ████████████████████████████████████████████████████████████

`Output    `      3 KiB 

`Reasoning `      2 KiB 
**Total:** 344 KiB


### 🎫 Token Usage

`Prompt      `    478,919 ████████████████████████████████████████████████████████████

`Completion  `     55,401 ███

`Cached      `    577,639 █████████████████████████████████████████████████████
**Total:** 039,320

_(577,639 cached)_

---

## 👤 Step 1: User Turn

### 📝 System Prompt

```
You are an expert Haskell and Bash programmer specialized in writing clean, idiomatic, and efficient Haskell code.
When writing Bash code:
- Avoid bash-specific logic but bash-ism are fine.
- Document required shell tools at the top of the bash script.
When writing Haskell code:
- Use modern Haskell practices (base >= 4.14)
- Prefer pure functions and explicit type signatures
- Use standard libraries when appropriate
- Add clear comments for complex logic
- Follow consistent formatting
Use list-files to explore the project structure, read-file to understand existing code, write-file to create or modify files, and check-code to verify your work.
Always compile code before considering the job done.
Your output format may be stored as commit message: hence the first line of your last message should be a summary line.
You have access to a tool which is a copy of yourself named io_prompt_agent_oneshot, use it when a task is well-scoped in a way that it would save tokens in your context window and not interfere with your work.
```

### 💬 User Query

# TUI: Navigate session by turn and fork from selected turn

Depends-on: 
Base-branch: gh-tui-turn-navigation
Final: false

## Overview

Add the ability to navigate through conversation/session turns and fork a new conversation starting from any selected turn. This enables users to "branch" their conversation history - useful for exploring alternative paths from a specific point without losing the original conversation.

## Background

### Current Session Structure

```haskell
-- From System/Agents/Session/Types.hs
data Session = Session
    { turns :: [Turn]
    , sessionId :: SessionId
    , forkedFromSessionId :: Maybe SessionId
    , turnId :: TurnId
    }

data Turn
    = UserTurn UserTurnContent (Maybe StepByteUsage)
    | LlmTurn LlmTurnContent (Maybe StepByteUsage)
```

The `forkedFromSessionId` field already exists but is currently unused in the TUI. This feature will leverage it to track the parent session.

### Forking Concept

Forking at turn N means:
1. Create a new Session with `forkedFromSessionId = Just originalSessionId`
2. Copy only turns 0 through N-1 (all turns before the selected one)
3. Start a new conversation with this truncated session
4. The original session remains untouched

## Feature Requirements

### Functional Requirements

1. **Enter Turn Navigation Mode**: Press Enter when focused on Conversation/Session view to enter turn-by-turn navigation
2. **Navigate Turns**: Use Up/Down arrows to move between turns (both UserTurn and LlmTurn)
3. **Visual Highlight**: Clearly indicate which turn is currently selected
4. **Fork from Turn**: Press F to fork a new conversation starting from the selected turn (keeping only earlier turns)
5. **Exit Navigation Mode**: Press Enter again (or Esc) to exit turn navigation and return to normal view

### State Requirements

Need to track:
- Whether turn navigation mode is active
- Which turn index is currently selected (if any)
- The original session being navigated

## Proposed UI Design

### Normal Mode (current)

```
┌─────────────────────────────────────────────────────────────┐
│ Conversation                                                 │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │ -----------------------                                 │ │
│ │ < What is the best approach for...                      │ │
│ │ + ...                                                   │ │
│ │                                                         │ │
│ │ -----------------------                                 │ │
│ │ < You could consider using...                           │ │
│ │ 🤔 Thinking...                                          │ │
│ │ [tool calls: 2]                                         │ │
│ │                                                         │ │
│ │ <user types new message here>                           │ │
│ └─────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────┘
```

### Turn Navigation Mode (new)

```
┌─────────────────────────────────────────────────────────────┐
│ Conversation - Turn Navigation (3/8) [Enter:exit F:fork]    │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │ -----------------------                                 │ │
│ │▶ < What is the best approach for...      ← SELECTED    │ │
│ │  + ...                                                  │ │
│ │                                                         │ │
│ │ -----------------------                                 │ │
│ │  < You could consider using...                          │ │
│ │  🤔 Thinking...                                         │ │
│ │  [tool calls: 2]                                        │ │
│ │                                                         │ │
│ │  -----------------------                                │ │
│ │  < Another turn content...                              │ │
│ └─────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────┘
```

### Fork Confirmation

When F is pressed, show a confirmation/status:

```
┌─────────────────────────────────────────────────────────────┐
│ Status: Forked at turn 3 - New conversation @agent-fork-3   │
└─────────────────────────────────────────────────────────────┘
```

## Implementation Plan

### 1. Add Navigation State

In `System/Agents/TUI/Types.hs`:

```haskell
-- | State for turn-by-turn navigation
data TurnNavigationState = TurnNavigationState
    { _navSession :: Session
    -- ^ The session being navigated
    , _navSelectedTurnIndex :: Int
    -- ^ Currently selected turn index (0-based)
    , _navTotalTurns :: Int
    -- ^ Total number of turns for display
    }
    deriving (Show)

data UIState = UIState
    { ...
    , _turnNavigation :: Maybe TurnNavigationState
    -- ^ When Just, we are in turn navigation mode
    }

makeLenses ''TurnNavigationState
```

### 2. Add Widget Name for Turn View

```haskell
data WidgetName
    = ...
    | TurnNavigationWidget
    -- ^ For viewport scrolling during turn navigation
    deriving (Show, Eq, Ord)
```

### 3. Update Rendering Functions

In `System/Agents/TUI/Render.hs`:

```haskell
-- | Render conversation/session with optional turn navigation
render_session :: Maybe Session -> Set ConversationId -> [Text] -> Maybe TurnNavigationState -> Widget N
render_session Nothing _ _ _ =
    vBox [txt "session not started yet"]
render_session (Just session) _ongoingConvs queuedMsgs mNavState =
    case mNavState of
        Nothing -> 
            -- Normal mode: render as before
            vBox $ [render_queued_messages queuedMsgs]
                ++ [render_session_usage session]
                ++ map render_turn (reverse $ zip [0..] $ reverse session.turns)
        Just navState ->
            -- Navigation mode: render with selection highlight
            render_turn_navigation session navState

-- | Render session in turn navigation mode
render_turn_navigation :: Session -> TurnNavigationState -> Widget N
render_turn_navigation session navState =
    let selectedIdx = navState ^. navSelectedTurnIndex
        totalTurns = navState ^. navTotalTurns
        header = txt $ "Turn Navigation (" <> show (selectedIdx + 1) <> "/" <> show totalTurns <> ")"
        turnsWithIndices = zip [0..] (reverse $ reverse session.turns)  -- Maintain chronological order
     in borderWithLabel header $
        viewport TurnNavigationWidget Both $
        vBox $ 
            [ txt "Enter: exit navigation  F: fork from here  Up/Down: navigate"
            , txt ""
            ]
            ++ map (render_navigable_turn selectedIdx) turnsWithIndices

-- | Render a single turn with selection indicator
render_navigable_turn :: Int -> (Int, Turn) -> Widget N
render_navigable_turn selectedIdx (idx, turn) =
    let isSelected = idx == selectedIdx
        selectionMarker = if isSelected then "▶ " else "  "
        turnWidget = render_turn (idx, turn)
     in if isSelected
            then withAttr selectedTurnAttr $ 
                hBox [txt selectionMarker, turnWidget]
            else hBox [txt selectionMarker, turnWidget]

-- Add new attribute for selected turn
selectedTurnAttr :: AttrName
selectedTurnAttr = attrName "selectedTurn"
```

Update attribute map:
```haskell
tui_appAttrMap _ =
    attrMap Vty.defAttr
        [ ...
        , (selectedTurnAttr, BrickUtil.bg Vty.blue `Vty.withStyle` Vty.bold)
        ]
```

### 4. Add Event Handlers

In `System/Agents/TUI/Event.hs`:

```haskell
tui_appHandleEvent tracer ev = do
    -- Check if we're in turn navigation mode
    mNavState <- use (tuiUI . turnNavigation)
    case mNavState of
        Just navState -> handleTurnNavigationEvent navState ev
        Nothing -> handleNormalEvent tracer ev

-- | Handle events when in turn navigation mode
handleTurnNavigationEvent :: TurnNavigationState -> BrickEvent N AppEvent -> EventM N TuiState ()
handleTurnNavigationEvent navState ev =
    case ev of
        -- Exit navigation mode
        VtyEvent (Vty.EvKey Vty.KEnter []) -> do
            tuiUI . turnNavigation .= Nothing
            showStatus StatusInfo "Exited turn navigation"
        
        -- Also allow Esc to exit
        VtyEvent (Vty.EvKey Vty.KEsc []) -> do
            tuiUI . turnNavigation .= Nothing
            showStatus StatusInfo "Exited turn navigation"
        
        -- Navigate up (to earlier turns)
        VtyEvent (Vty.EvKey Vty.KUp []) -> do
            let currentIdx = navState ^. navSelectedTurnIndex
                newIdx = max 0 (currentIdx - 1)
            tuiUI . turnNavigation . _Just . navSelectedTurnIndex .= newIdx
            -- Scroll to keep selection visible
            vScrollToElement (viewportScroll TurnNavigationWidget) $
                txt $ "turn-" <> show newIdx
        
        -- Navigate down (to later turns)
        VtyEvent (Vty.EvKey Vty.KDown []) -> do
            let currentIdx = navState ^. navSelectedTurnIndex
                maxIdx = (navState ^. navTotalTurns) - 1
                newIdx = min maxIdx (currentIdx + 1)
            tuiUI . turnNavigation . _Just . navSelectedTurnIndex .= newIdx
            -- Scroll to keep selection visible
            vScrollToElement (viewportScroll TurnNavigationWidget) $
                txt $ "turn-" <> show newIdx
        
        -- Fork at current turn
        VtyEvent (Vty.EvKey (Vty.KChar 'f') []) -> do
            handleForkAtTurn navState
        
        -- Fork at current turn (uppercase F)
        VtyEvent (Vty.EvKey (Vty.KChar 'F') []) -> do
            handleForkAtTurn navState
        
        _ -> pure ()

-- | Fork a new conversation at the selected turn
handleForkAtTurn :: TurnNavigationState -> EventM N TuiState ()
handleForkAtTurn navState = do
    let session = navState ^. navSession
        selectedIdx = navState ^. navSelectedTurnIndex
        originalSessionId = session.sessionId
        
    -- Create forked session with only turns 0 to selectedIdx-1
    let turnsToKeep = take selectedIdx session.turns
    
    -- Generate new session ID and turn ID
    newSessionId <- liftIO newSessionId
    newTurnId <- liftIO newTurnId
    
    let forkedSession = Session
            { turns = turnsToKeep
            , sessionId = newSessionId
            , forkedFromSessionId = Just originalSessionId
            , turnId = newTurnId
            }
    
    -- Create a new conversation with this session
    -- (similar to handleRestoredConversation but with forked session)
    mAgent <- use (tuiUI . agentList . to listSelectedElement)
    case mAgent of
        Just (_, baseTuiAgent) -> do
            -- Start conversation with forked session
            runConversation tracer baseTuiAgent forkedSession
            
            -- Show status
            showStatus StatusInfo $ 
                "Forked at turn " <> show selectedIdx <> 
                " - New conversation " <> show newSessionId
            
            -- Exit navigation mode
            tuiUI . turnNavigation .= Nothing
            
        Nothing -> 
            showStatus StatusError "No agent selected for forked conversation"

-- | Handle normal (non-navigation) events
handleNormalEvent :: Tracer IO Trace -> BrickEvent N AppEvent -> EventM N TuiState ()
handleNormalEvent tracer ev = do
    case ev of
        ... -- existing handlers
        
        -- Enter turn navigation mode
        VtyEvent (Vty.EvKey Vty.KEnter []) -> do
            mSession <- getFocusedSession
            case mSession of
                Just session | not (null session.turns) -> do
                    let navState = TurnNavigationState
                            { _navSession = session
                            , _navSelectedTurnIndex = length session.turns - 1  -- Start at most recent
                            , _navTotalTurns = length session.turns
                            }
                    tuiUI . turnNavigation .= Just navState
                    showStatus StatusInfo "Enter: turn navigation (Up/Down to navigate, F to fork)"
                _ -> 
                    showStatus StatusWarning "No session or empty session to navigate"
        
        ... -- rest of existing handlers
```

### 5. Update Conversation Rendering to Support Navigation

Modify `render_conversationView` to pass navigation state:

```haskell
render_conversationView :: TuiState -> Widget N
render_conversationView st =
    borderWithFocus st ConversationViewWidget "Conversation" content
  where
    content =
        case listSelectedElement (st ^. tuiUI . conversationList) of
            Nothing -> txt "No conversation selected"
            Just (_, conv) ->
                let queuedMsgs = getQueuedMessages st conv
                    mNavState = st ^. tuiUI . turnNavigation
                 in viewport ConversationViewWidget Both $ 
                    render_session (conversationSession conv) 
                                   (st ^. tuiUI . ongoingConversations) 
                                   queuedMsgs
                                   mNavState
```

### 6. Update Help Content

Add to `defaultHelpContent`:

```haskell
, "Session Navigation & Forking:"
, "  Enter        - Enter turn navigation mode (when on conversation)"
, "  Up/Down      - Navigate between turns (in navigation mode)"
, "  F            - Fork conversation at selected turn"
, "  Enter/Esc    - Exit turn navigation mode"
```

## Files to Modify

| File | Changes |
|------|---------|
| `src/System/Agents/TUI/Types.hs` | Add `TurnNavigationState` type; add `_turnNavigation` to `UIState`; add `TurnNavigationWidget` to `WidgetName` |
| `src/System/Agents/TUI/Render.hs` | Update `render_session` signature; add `render_turn_navigation`, `render_navigable_turn`; add `selectedTurnAttr` |
| `src/System/Agents/TUI/Event.hs` | Add `handleTurnNavigationEvent`, `handleForkAtTurn`; modify main handler to check navigation mode; update `defaultHelpContent` |

## Data Types (for reference)

```haskell
-- Turn types
-- From System/Agents/Session/Types.hs
data Turn
    = UserTurn UserTurnContent (Maybe StepByteUsage)
    | LlmTurn LlmTurnContent (Maybe StepByteUsage)

data UserTurnContent = UserTurnContent
    { userPrompt :: SystemPrompt
    , userTools :: [SystemTool]
    , userQuery :: Maybe UserQuery
    , userToolResponses :: [(LlmToolCall, UserToolResponse)]
    }

data LlmTurnContent = LlmTurnContent
    { llmResponse :: LlmResponse
    , llmToolCalls :: [LlmToolCall]
    }

-- Session with forking support
data Session = Session
    { turns :: [Turn]
    , sessionId :: SessionId
    , forkedFromSessionId :: Maybe SessionId  -- Already exists!
    , turnId :: TurnId
    }
```

## User Flow Example

1. User has a conversation with 5 turns (alternating User/LLM)
2. User realizes turn 3 (LLM response) wasn't what they wanted
3. User presses `Enter` to enter turn navigation mode
4. User presses `Up` twice to navigate to turn 3
5. User presses `F` to fork
6. System creates new conversation with only turns 0-2
7. User continues the new conversation with a different approach
8. Original conversation remains in history tab

## Edge Cases

1. **Empty session**: Enter key should show warning, not enter navigation mode
2. **Fork at turn 0**: Creates empty session (valid but unusual)
3. **Fork at last turn**: Effectively duplicates the session
4. **Agent change**: If user selects different agent before forking, fork uses new agent
5. **Navigation while ongoing**: Should be allowed - user can review history even during processing

## Testing Considerations

1. **Navigation**: Test Up/Down cycling through all turns
2. **Fork at various positions**: First turn, middle turn, last turn
3. **Session integrity**: Verify original session unchanged after fork
4. **Fork metadata**: Verify `forkedFromSessionId` is set correctly
5. **Empty sessions**: Verify graceful handling
6. **Exit modes**: Both Enter and Esc should exit navigation
7. **Visual feedback**: Selected turn clearly highlighted

## Tradeoffs & Design Decisions

1. **Enter key for activation**: Reuses an existing key in a context-sensitive way. In normal mode it enters navigation; in navigation mode it exits.

2. **F key for fork**: Simple and mnemonic. Could conflict with F1-F12 but F alone is safe.

3. **Start at most recent turn**: Users typically want to fork from recent history, not the beginning.

4. **No confirmation dialog**: Forking is non-destructive (original preserved), so immediate action is acceptable.

5. **Fork uses current agent selection**: User can change agent before forking to "replay" with different agent.

## Future Enhancements (Out of Scope)

- **Compare forks**: Side-by-side view of original and forked conversations
- **Merge forks**: Combine insights from multiple forks
- **Branch visualization**: Tree view of all forks from a session
- **Named forks**: Allow users to name forked sessions
- **Keyboard shortcut for quick fork**: Direct key to fork from last turn without entering navigation

---
From #437



### 🛠️ Available Tools

- **bash_grep-files** (`bash_grep-files`)
  - Description: Search for a pattern in files using 'git grep'
- **bash_read-file** (`bash_read-file`)
  - Description: Read the contents of a file
- **bash_check-code** (`bash_check-code`)
  - Description: Check Haskell code by compiling or testing
- **bash_list-files** (`bash_list-files`)
  - Description: List all files in the project
- **bash_write-file** (`bash_write-file`)
  - Description: Write content to a file. Optionally compiles or tests the code after writing. Use 'none' for the check action when making multiple edits that are not yet final, to avoid redundant compilation.
- **bash_run_agents_exe** (`bash_run_agents_exe`)
  - Description: Runs agents-exe with specified arguments. Useful for testing CLI commands like init, check, run, describe, and running repro cases when bug with specific command arise.
- **io_prompt_agent_oneshot** (`io_prompt_agent_oneshot`)
  - Description: asks a prompt to the expert agent: oneshot

---

## 🤖 Step 2: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

The user wants me to implement a TUI feature for navigating session turns and forking from selected turns. Let me start by exploring the project structure to understand the codebase.
</details>

### 💬 Response

I'll start by exploring the project structure to understand the existing codebase, then implement the turn navigation and forking feature.

### 🔧 Tool Calls

- **bash_list-files**

---

## 👤 Step 3: User Turn



---

## 🤖 Step 4: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

Let me explore the TUI files to understand the current implementation:
</details>

### 💬 Response

Let me explore the TUI source files to understand the current implementation:

### 🔧 Tool Calls

- **bash_read-file**

- **bash_read-file**

- **bash_read-file**

---

## 👤 Step 5: User Turn



---

## 🤖 Step 6: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

Now I understand the codebase structure. Let me also look at the Session types to understand the Session and Turn structures:
</details>

### 💬 Response

Let me also look at the Session types to understand the Session and Turn structures better:

### 🔧 Tool Calls

- **bash_read-file**

- **bash_read-file**

---

## 👤 Step 7: User Turn



---

## 🤖 Step 8: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

Now I have a good understanding of the codebase. Let me implement the turn navigation and forking feature. I'll need to modify three files:

1. `System.Agents.TUI.Types` - Add `TurnNavigationState` type, add `_turnNavigation` to `UIState`, and add `TurnNavigationWidget` to `WidgetName`
2. `System.Agents.TUI.Render` - Update `render_session` to support navigation mode, add `render_turn_navigation`, `render_navigable_turn`, and add `selectedTurnAttr`
3. `System.Agents.TUI.Event` - Add handlers for turn navigation events

Let me start implementing these changes:
</details>

### 💬 Response

Now I have a good understanding of the codebase. Let me implement the turn navigation and forking feature. I'll start with the types:

### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 9: User Turn



---

## 🤖 Step 10: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

The types file compiled successfully. Now let me update the Render.hs file to add support for turn navigation:
</details>

### 💬 Response

The types compiled successfully. Now let me update the Render.hs file to add turn navigation support:

### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 11: User Turn



---

## 🤖 Step 12: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

Now I need to update the Event.hs file to add the turn navigation event handlers. This is the most complex part. Let me update the file:
</details>

### 💬 Response

Now let me update the Event.hs file to add the turn navigation event handlers:

### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 13: User Turn



---

## 🤖 Step 14: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

I need to add the `(^.)` operator to the imports from Control.Lens. Let me fix that:
</details>

### 💬 Response

I need to add the `(^.)` operator to the Lens imports:

### 🔧 Tool Calls

- **bash_read-file**

---

## 👤 Step 15: User Turn



---

## 🤖 Step 16: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

I need to fix the import statement to add the `(^.)` operator. Let me update the import:
</details>

### 💬 Response



### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 17: User Turn



---

## 🤖 Step 18: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

The issue is that `_Just` requires a Monoid instance when used this way. Let me use a different approach - we can pattern match on the Just value or use a different way to access it since we know it will be Just when we're in handleTurnNavigationEvent.
</details>

### 💬 Response

I need to fix the way we access the turnNavigation state. Let me use a safer approach:

### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 19: User Turn



---

## 🤖 Step 20: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

I need to add an underscore prefix to the unused parameters or remove them:
</details>

### 💬 Response

Let me fix the unused variable warnings:

### 🔧 Tool Calls

- **bash_grep-files**

---

## 👤 Step 21: User Turn



---

## 🤖 Step 22: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

The issue is that handleSessionViewEvent receives the tracer parameter but doesn't use it. I should add an underscore prefix or remove it from the parameter list. Let me update both handler functions to use underscores:
</details>

### 💬 Response



### 🔧 Tool Calls

- **bash_write-file**

---

## 👤 Step 23: User Turn



---

## 🤖 Step 24: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

The code compiled successfully. Now let me run the tests to make sure everything works correctly:
</details>

### 💬 Response

All code compiles successfully. Let me run the tests to make sure everything works:

### 🔧 Tool Calls

- **bash_check-code**

---

## 👤 Step 25: User Turn



---

## 🤖 Step 26: LLM Turn

### 💭 Thinking Process

<details>
<summary>Click to expand reasoning</summary>

All 599 tests passed! The implementation is complete. Let me provide a summary of what was implemented.
</details>

### 💬 Response

All 599 tests passed. The turn navigation and forking feature has been successfully implemented.

## Summary

I've implemented the TUI turn navigation and forking feature as specified. Here's what was added:

### Files Modified

1. **`src/System/Agents/TUI/Types.hs`**:
   - Added `TurnNavigationWidget` to `WidgetName` enum
   - Added `TurnNavigationState` type with lenses (`navSession`, `navSelectedTurnIndex`, `navTotalTurns`)
   - Added `_turnNavigation :: Maybe TurnNavigationState` field to `UIState`
   - Initialized `_turnNavigation = Nothing` in `initUIState`

2. **`src/System/Agents/TUI/Render.hs`**:
   - Added `selectedTurnAttr` attribute for highlighting selected turns
   - Modified `render_session` to accept and handle `Maybe TurnNavigationState`
   - Added `render_turn_navigation` function for navigation mode UI
   - Added `render_navigable_turn` function for rendering individual turns with selection indicator
   - Updated `render_conversationView` and `render_sessionView` to pass navigation state
   - Added `selectedTurnAttr` to attribute map (blue background with bold)

3. **`src/System/Agents/TUI/Event.hs`**:
   - Added turn navigation help content to `defaultHelpContent`
   - Modified `tui_appHandleEvent` to check for navigation mode first
   - Added `handleTurnNavigationEvent` function that handles:
     - `Enter` / `Esc` - Exit navigation mode
     - `Up` / `Down` - Navigate between turns
     - `f` / `F` - Fork at selected turn
   - Added `handleForkAtTurn` function that:
     - Creates a new session with only turns before the selected index
     - Sets `forkedFromSessionId` to the original session ID
     - Starts a new conversation with the forked session
   - Modified `handleSessionViewEvent` and `handleConversationViewEvent` to enter navigation mode on Enter key
   - Updated `getFocusedSession` to also check session list

### User Flow

1. **Enter Navigation Mode**: Press `Enter` when focused on Conversation view or Session view
2. **Navigate Turns**: Use `Up`/`Down` arrows to move between turns (starts at most recent)
3. **Fork**: Press `F` to create a new conversation starting from the selected turn (keeps only earlier turns)
4. **Exit**: Press `Enter` or `Esc` to exit navigation mode

### Key Design Decisions

- **Forking at turn N** keeps turns 0 through N-1 (all turns before the selected one)
- **Original session** remains untouched - forking is non-destructive
- **Selected agent** is used for the forked conversation (can be changed before forking)
- **Visual indicator**: Selected turn shows "▶" prefix with blue highlight
- **Turn counter**: Shows "(X/Y)" in the border label during navigation mode
