start of hooks engine (#13276)

(Experimental)

This PR adds a first MVP for hooks, with SessionStart and Stop

The core design is:

- hooks live in a dedicated engine under codex-rs/hooks
- each hook type has its own event-specific file
- hook execution is synchronous and blocks normal turn progression while
running
- matching hooks run in parallel, then their results are aggregated into
a normalized HookRunSummary

On the AppServer side, hooks are exposed as operational metadata rather
than transcript-native items:

- new live notifications: hook/started, hook/completed
- persisted/replayed hook results live on Turn.hookRuns
- we intentionally did not add hook-specific ThreadItem variants

Hooks messages are not persisted, they remain ephemeral. The context
changes they add are (they get appended to the user's prompt)

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -166,6 +166,7 @@ impl ThreadHistoryBuilder {
             EventMsg::ExitedReviewMode(payload) => self.handle_exited_review_mode(payload),
             EventMsg::ItemStarted(payload) => self.handle_item_started(payload),
             EventMsg::ItemCompleted(payload) => self.handle_item_completed(payload),
+            EventMsg::HookStarted(_) | EventMsg::HookCompleted(_) => {}
             EventMsg::Error(payload) => self.handle_error(payload),
             EventMsg::TokenCount(_) => {}
             EventMsg::ThreadRolledBack(payload) => self.handle_thread_rollback(payload),