Extract hooks into dedicated crate (#11311)

Summary
- move `core/src/hooks` implementation into a new `codex-hooks` crate
with its own manifest
- update `codex-rs` workspace and `codex-core` crate to depend on the
extracted `hooks` crate and wire up the shared APIs
- ensure references, modules, and lockfile reflect the new crate layout

Testing
- Not run (not requested)

codex-rs/hooks/src/lib.rs

@@ -0,0 +1,13 @@
+mod registry;
+mod types;
+mod user_notification;
+
+pub use registry::Hooks;
+pub use registry::command_from_argv;
+pub use types::Hook;
+pub use types::HookEvent;
+pub use types::HookEventAfterAgent;
+pub use types::HookOutcome;
+pub use types::HookPayload;
+pub use user_notification::legacy_notify_json;
+pub use user_notification::notify_hook;