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/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__deltas_then_same_final_message_are_rendered_snapshot.snap

@@ -0,0 +1,6 @@
+---
+source: tui/src/chatwidget/tests.rs
+assertion_line: 9494
+expression: combined
+---
+

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__hook_events_render_snapshot.snap

@@ -0,0 +1,10 @@
+---
+source: tui/src/chatwidget/tests.rs
+assertion_line: 8586
+expression: combined
+---
+• Running SessionStart hook: warming the shell
+
+SessionStart hook (completed)
+  warning: Heads up from the hook
+  hook context: Remember the startup checklist.