## Description
This PR moves hook prompts onto the canonical `TurnItem` lifecycle in
core.
Stop hooks now record their `ResponseItem` through the existing
lifecycle path, which emits `ItemStarted` and `ItemCompleted`.
App-server consumes those events directly instead of deriving a hook
prompt from `RawResponseItem`.
## Why
Hook prompts were the only `ThreadItem` app-server synthesized from
`RawResponseItem`. This brings them in line with other core-owned turn
items while preserving legacy rollout replay.
## What changed
- Route stop-hook prompts through
`record_response_item_and_emit_turn_item`.
- Materialize canonical hook prompts in `ThreadHistoryBuilder`.
- Remove `RawResponseItem` to `ThreadItem` synthesis while preserving
legacy rollout replay.
- Add focused coverage for lifecycle emission and canonical and legacy
history materialization.
Owen Lin
ba5dd1fd3a