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/common.rs

@@ -828,7 +828,9 @@ server_notification_definitions! {
     ThreadNameUpdated => "thread/name/updated" (v2::ThreadNameUpdatedNotification),
     ThreadTokenUsageUpdated => "thread/tokenUsage/updated" (v2::ThreadTokenUsageUpdatedNotification),
     TurnStarted => "turn/started" (v2::TurnStartedNotification),
+    HookStarted => "hook/started" (v2::HookStartedNotification),
     TurnCompleted => "turn/completed" (v2::TurnCompletedNotification),
+    HookCompleted => "hook/completed" (v2::HookCompletedNotification),
     TurnDiffUpdated => "turn/diff/updated" (v2::TurnDiffUpdatedNotification),
     TurnPlanUpdated => "turn/plan/updated" (v2::TurnPlanUpdatedNotification),
     ItemStarted => "item/started" (v2::ItemStartedNotification),