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/mcp-server/src/codex_tool_runner.rs

@@ -359,6 +359,8 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::EnteredReviewMode(_)
                     | EventMsg::ItemStarted(_)
                     | EventMsg::ItemCompleted(_)
+                    | EventMsg::HookStarted(_)
+                    | EventMsg::HookCompleted(_)
                     | EventMsg::AgentMessageContentDelta(_)
                     | EventMsg::ReasoningContentDelta(_)
                     | EventMsg::ReasoningRawContentDelta(_)