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/hooks/src/lib.rs

@@ -1,10 +1,21 @@
+mod engine;
+pub mod events;
+mod legacy_notify;
 mod registry;
+mod schema;
 mod types;
-mod user_notification;
 
+pub use events::session_start::SessionStartOutcome;
+pub use events::session_start::SessionStartRequest;
+pub use events::session_start::SessionStartSource;
+pub use events::stop::StopOutcome;
+pub use events::stop::StopRequest;
+pub use legacy_notify::legacy_notify_json;
+pub use legacy_notify::notify_hook;
 pub use registry::Hooks;
 pub use registry::HooksConfig;
 pub use registry::command_from_argv;
+pub use schema::write_schema_fixtures;
 pub use types::Hook;
 pub use types::HookEvent;
 pub use types::HookEventAfterAgent;
@@ -15,5 +26,3 @@ pub use types::HookResult;
 pub use types::HookToolInput;
 pub use types::HookToolInputLocalShell;
 pub use types::HookToolKind;
-pub use user_notification::legacy_notify_json;
-pub use user_notification::notify_hook;