feat(hooks): include the final assistant message in Stop payload

While testing Rocky's Kimi Code integration with Kimi Code CLI v0.29.0 on macOS, I observed that the Stop hook payload contains only:

{hook_event_name, session_id, cwd, stop_hook_active}

Because it contains no assistant text, an integration using the Stop payload cannot show what Kimi said when the turn ended. In one observed case, Kimi ended with a question for the user, but the UI could only display "done."

The text is present in the session's wire.jsonl as a content.part event, but using it requires parsing the on-disk session log instead of receiving the text through the hook.

Could Stop include the final assistant message, for example:

"last_assistant_message": "…"

Claude Code exposes this field in its Stop hook and recommends using it instead of reading transcript_path, because its transcript is written asynchronously and can lag. A similar field would give Kimi integrations a direct, hook-supported way to display the final response.

Start by tracing how the Stop hook payload is assembled and compare it with the final assistant content recorded as a content.part event in the session's wire.jsonl. Check whether existing hook tests cover Stop payload fields. Done means the Stop payload exposes the final assistant message reliably for integrations without requiring session-log parsing.