Move skills watcher to app-server (#21287)

## Why

Skills update notifications are app-server API behavior, but the watcher
lived in `codex-core` and surfaced through
`EventMsg::SkillsUpdateAvailable`. Moving the watcher out keeps core
focused on thread execution and lets app-server own both cache
invalidation and the `skills/changed` notification.

## What changed

- Added an app-server-owned skills watcher that watches local skill
roots, clears the shared skills cache, and emits `skills/changed`
directly.
- Registers skill watches from the common app-server thread listener
attach path, including direct starts, resumes, and app-server-observed
child or forked threads.
- Stores the `WatchRegistration` on `ThreadState`, so listener
replacement, thread teardown, idle unload, and app-server shutdown
deregister by dropping the RAII guard.
- Removed `EventMsg::SkillsUpdateAvailable`, the core watcher, and the
old core live-reload test.
- Extended the app-server skills change test to verify a cached skills
list is refreshed after a filesystem change without forcing reload.

## Validation

- `cargo check -p codex-core -p codex-app-server -p codex-mcp-server -p
codex-rollout -p codex-rollout-trace`
- `cargo test -p codex-app-server
skills_changed_notification_is_emitted_after_skill_change`

codex-rs/app-server/src/thread_state.rs

@@ -7,6 +7,7 @@ use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnError;
 use codex_core::CodexThread;
 use codex_core::ThreadConfigSnapshot;
+use codex_core::file_watcher::WatchRegistration;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::RolloutItem;
@@ -77,6 +78,7 @@ pub(crate) struct ThreadState {
     listener_command_tx: Option<mpsc::UnboundedSender<ThreadListenerCommand>>,
     current_turn_history: ThreadHistoryBuilder,
     listener_thread: Option<Weak<CodexThread>>,
+    watch_registration: WatchRegistration,
 }
 
 impl ThreadState {
@@ -91,6 +93,7 @@ impl ThreadState {
         &mut self,
         cancel_tx: oneshot::Sender<()>,
         conversation: &Arc<CodexThread>,
+        watch_registration: WatchRegistration,
     ) -> (mpsc::UnboundedReceiver<ThreadListenerCommand>, u64) {
         if let Some(previous) = self.cancel_tx.replace(cancel_tx) {
             let _ = previous.send(());
@@ -99,6 +102,7 @@ impl ThreadState {
         let (listener_command_tx, listener_command_rx) = mpsc::unbounded_channel();
         self.listener_command_tx = Some(listener_command_tx);
         self.listener_thread = Some(Arc::downgrade(conversation));
+        self.watch_registration = watch_registration;
         (listener_command_rx, self.listener_generation)
     }
 
@@ -109,6 +113,7 @@ impl ThreadState {
         self.listener_command_tx = None;
         self.current_turn_history.reset();
         self.listener_thread = None;
+        self.watch_registration = WatchRegistration::default();
     }
 
     pub(crate) fn set_experimental_raw_events(&mut self, enabled: bool) {