feat(hooks): Stop event — fires once per response when yielding to user

[emal-avala]

Summary

Adds a Stop hook event that fires exactly once per user-visible response, right after the final PostTurn and before control returns to the REPL / one-shot exit. PostTurn fires once per LLM round-trip — a multi-step turn with N tool calls fires PostTurn N+1 times, which is too noisy for auto-commit / transcript-shipping / desktop-notification workflows that want "agent finished; now react."

Context payload

{
  "session_id": "...",
  "turn_count": 3,
  "last_assistant_message": "..."
}

last_assistant_message is extracted via a new last_assistant_text helper that walks the message history backward, skipping pure tool-use assistant turns so the hook sees the actual user-visible reply — not the internal "I'm about to call Bash" step.

Wiring

• HookEvent::Stop added to the schema enum with snake_case serde
• fire_stop_hooks(last_assistant_message: Option<&str>) async method on QueryEngine
• Called from run_turn_with_sink's no-more-tools completion branch, right after PostTurn
• HOOK_EVENT_CATALOG, format_hook_event, parse_hook_event updated
• hooks/mod.rs module docs updated

Test plan

• cargo clippy --workspace --tests --no-deps -- -D warnings — clean
• cargo test -p agent-code-lib --lib hooks::tests — 6 pass
• cargo test -p agent-code-lib --lib query::tests::last_assistant_text — 4 pass
• cargo test -p agent-code-lib --lib config::schema::tests::hook_event_serde_roundtrip — 10 pass
• cargo test -p agent-code --bin agent commands::tests::parse_hook_event_accepts_stop — pass

6 new tests:

1. Schema serde round-trip (stop ↔ "stop")
2. run_hooks dispatches for Stop
3. parse_hook_event accepts "stop" / "Stop" / "STOP"
4. last_assistant_text: empty history returns None
5. last_assistant_text: returns most recent assistant text / joins multiple blocks
6. last_assistant_text: skips pure-tool-use assistant messages

Follow-up

SubagentStop needs the parent's HookRegistry plumbed into ToolContext (our subagents are subprocesses, so the parent fires on child exit). Separate PR once this lands.

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