Reapply "Move skills watcher to app-server" (#21652)

## Why

PR #21460 reverted the earlier move of skills change watching from
`codex-core` into app-server. This reapplies that boundary change so
app-server owns client-facing `skills/changed` notifications and core no
longer carries the watcher.

## What

- Restore the app-server `SkillsWatcher` and register it from thread
listener setup.
- Remove the core-owned skills watcher and its core live-reload
integration surface.
- Restore app-server coverage for `skills/changed` notifications after a
watched skill file changes.

## Validation

- `cargo test -p codex-app-server --test all
suite::v2::skills_list::skills_changed_notification_is_emitted_after_skill_change
-- --exact --nocapture`
- `cargo test -p codex-core --lib --no-run`

codex-rs/protocol/src/protocol.rs

@@ -1401,9 +1401,6 @@ pub enum EventMsg {
     /// List of voices supported by realtime conversation streams.
     RealtimeConversationListVoicesResponse(RealtimeConversationListVoicesResponseEvent),
 
-    /// Notification that skill data may have been updated and clients may want to reload.
-    SkillsUpdateAvailable,
-
     PlanUpdate(UpdatePlanArgs),
 
     TurnAborted(TurnAbortedEvent),