# `SessionMode` deserialization silently falls back on unknown values; typos cause semantic drift

**Severity:** Medium
**Category:** Input validation
**Labels:** `validation`, `config`, `medium`
**Verification (re-audit 2026-05-18): DISPUTED.** serde's default behavior for `#[derive(Deserialize)] enum` without `#[serde(other)]` is to **error hard** on unknown variants — it does **not** silently fall back to `#[default]`. The `#[default]` attribute only kicks in for `#[serde(default)]` on container/field initialization, not as a fallback for unknown tagged-variant strings. So `session_mode = "New"` (typo) actually errors at agent.toml load, with a clear "unknown variant" message. The audit's framing contradicts serde semantics. The recommendation to extend `info!(session_mode=resolved, ...)` logging to the manifest-load path is still useful for visibility, but the bug itself does not exist.

## Affected files
- `crates/librefang-types/src/agent.rs:450-458` (`SessionMode`)
- `crates/librefang-kernel/src/kernel/cron_tick.rs:170` (`effective_session_mode`)
- `crates/librefang-types/src/agent.rs:892` (`Trigger.session_mode: Option<SessionMode>`)

## Description

`#[serde(rename_all = "snake_case")] #[default] Persistent` lacks `#[serde(other)]`; combined with `Option<SessionMode>` + `#[serde(default)]`:

- `agent.toml` containing `session_mode = "presistent"` (typo) errors hard — acceptable;
- `session_mode = ""` may collapse to `None` → defaults to `Persistent`;
- `session_mode = "New"` (case-sensitive — snake_case wants `"new"`) is unknown → defaults to `Persistent`.

An operator who intended `"new"` writes `"New"`, silently gets `Persistent` semantics, and runs straight into CLAUDE.md's warning that "concurrent writes to a single persistent session are undefined."

## Recommendation

Custom `deserialize_with` that returns `ConfigError` rather than the default on unknown values:

```rust
fn deserialize_session_mode<'de, D>(d: D) -> Result<SessionMode, D::Error>
where D: Deserializer<'de> {
    let s = String::deserialize(d)?;
    match s.as_str() {
        "persistent" => Ok(SessionMode::Persistent),
        "new" => Ok(SessionMode::New),
        other => Err(D::Error::custom(format!("unknown session_mode: {other}"))),
    }
}
```

Extend `cron_tick.rs:179-195`'s `info!(session_mode=resolved, ...)` to the manifest-load path so operators see the resolved value at startup.
