## Description

Extend the three review skills (impl-review, plan-review, epic-review) so they recognize `copilot` as a valid backend and invoke `flowctl copilot ...` wrappers. Analogous to their existing `rp` / `codex` branches.

**Size:** M
**Files:**
- `plugins/flow-next/skills/flow-next-impl-review/SKILL.md`
- `plugins/flow-next/skills/flow-next-impl-review/workflow.md`
- `plugins/flow-next/skills/flow-next-plan-review/SKILL.md`
- `plugins/flow-next/skills/flow-next-plan-review/workflow.md`
- `plugins/flow-next/skills/flow-next-epic-review/SKILL.md`
- `plugins/flow-next/skills/flow-next-epic-review/workflow.md`

## Approach

For each of the three review skills:

1. **Extend the backend enum** in SKILL.md front matter / intro text wherever `rp|codex|none` appears. Add `copilot`. docs-gap-scout listed exact lines:
   - `flow-next-impl-review/SKILL.md` lines 14, 24-26, 42, 45-46, 50
   - `flow-next-impl-review/workflow.md` lines 5, 21, 24-25, 29
   - `flow-next-plan-review/SKILL.md` lines 24-26, 32-35, 43, 47, 51
   - `flow-next-plan-review/workflow.md` lines 21, 25, 29
   - `flow-next-epic-review/SKILL.md` lines 24-26, 32-35
   - `flow-next-epic-review/workflow.md` lines 25, 29

2. **Add the copilot workflow branch** in each workflow.md. Mirror the existing codex branch (usually starts around line 21 "If backend is codex:"). The copilot block wraps `flowctl copilot <impl|plan|completion>-review` with the same env-var preamble style as codex.
   - docs-gap-scout identified insertion points at ~line 335-349 (impl), ~line 359-372 (plan), ~line 365-380 (epic).
   - **Flag shape (task-3 verified):** `impl-review [task] --base <branch> [--focus <x>] [--receipt <path>] [--json]`; `plan-review <epic> --files <a,b,c> [--base <branch>] [--receipt <path>] [--json]`; `completion-review <epic> [--base <branch>] [--receipt <path>] [--json]`. Same as codex minus `--sandbox`.
   - **Model + effort are env-only (task-3 finding):** `flowctl copilot *-review` has NO `--model` or `--effort` CLI flags. Set via `FLOW_COPILOT_MODEL` / `FLOW_COPILOT_EFFORT` env vars in the preamble (cascade: env > internal default). Resolved values land in receipt (`model`, `effort` keys). Do not invent CLI flags in skill examples.
<!-- Updated by plan-sync: task 3 added _resolve_copilot_model_effort() env-cascade; no CLI flags for model/effort -->
<!-- Updated by plan-sync: task 3 confirmed argparse shape (task/epic positional, --base, --files plan-only, --focus impl-only, --receipt, --json) -->

**Re-review note (task-3 finding):** `flowctl copilot *-review` only resumes the copilot session when the prior receipt at `--receipt` has `mode == "copilot"`. Cross-backend receipts (e.g., codex → copilot switch on same path) start a fresh session. Skill prose should not assume session continuity across backend switches.
<!-- Updated by plan-sync: task 3 guards session resume on prior-receipt mode==copilot to prevent cross-backend UUID leakage -->

3. **Update the "Which backend?" advice blocks** (impl-review/SKILL.md `## Backend Selection` or similar) — add a one-line summary for copilot: "Uses GitHub Copilot CLI; cross-platform; supports Claude Opus/Sonnet/Haiku 4.5 and GPT-5.2 families via a Copilot subscription." Keep it terse.

4. **Help-text updates**: any skill that surfaces `--review=rp|codex|export|none` (impl-review SKILL.md typically) must add `|copilot` to the pipe-delimited list.

Do NOT edit `plugins/flow-next/codex/skills/**` — those are regenerated by `scripts/sync-codex.sh` in task 7.

## Investigation targets

**Required:**
- `plugins/flow-next/skills/flow-next-impl-review/workflow.md:9-81` — existing codex workflow block (mirror shape)
- `plugins/flow-next/skills/flow-next-plan-review/workflow.md:9-85` — same
- `plugins/flow-next/skills/flow-next-epic-review/workflow.md:13-75` — same
- `plugins/flow-next/skills/flow-next-impl-review/SKILL.md` — full file for backend-selection prose

**Optional:**
- `plugins/flow-next/skills/flow-next-setup/workflow.md:150-349` — for reference on how other skills detect backends (setup skill changes happen in task 6)

## Acceptance

- [ ] Every occurrence of `rp|codex|none` (or `rp`/`codex`/`none` list) in the three review skills includes `copilot`
- [ ] Each `workflow.md` has a Copilot branch that invokes `flowctl copilot <impl|plan|completion>-review` with correct args
- [ ] Skill examples / "Quick commands" sections show a copilot invocation
- [ ] When `REVIEW_BACKEND=copilot` (from `flowctl review-backend`), each skill routes to the copilot branch
- [ ] No edits to generated codex mirror files (`plugins/flow-next/codex/**`)
- [ ] Skill files still render cleanly when read by the skill system (no broken markdown / front matter errors)

## Done summary
Extended impl-review, plan-review, and epic-review skills (SKILL.md + workflow.md each) to branch on the copilot backend alongside rp and codex: backend-selection enums, per-backend Critical Rules, full Copilot Backend Workflow blocks mirroring codex shape (env-only model/effort, receipt-gated session resume with mode==copilot guard), and copilot-specific anti-patterns. No Python or generated codex/ files touched; smoke_test.sh runs 52/52 green.
## Evidence
- Commits: 18840a8b3173d4d031950ebefa73094b5c7809d1
- Tests: plugins/flow-next/scripts/smoke_test.sh (52/52 passed)
- PRs: