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/core/src/session/mod.rs

@@ -113,6 +113,7 @@ use codex_protocol::protocol::ThreadSource;
 use codex_protocol::protocol::TurnAbortReason;
 use codex_protocol::protocol::TurnContextItem;
 use codex_protocol::protocol::TurnContextNetworkItem;
+use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_protocol::protocol::W3cTraceContext;
 use codex_protocol::request_permissions::PermissionGrantScope;
 use codex_protocol::request_permissions::RequestPermissionProfile;
@@ -283,8 +284,6 @@ use crate::rollout::map_session_init_error;
 use crate::session_startup_prewarm::SessionStartupPrewarmHandle;
 use crate::shell;
 use crate::shell_snapshot::ShellSnapshot;
-use crate::skills_watcher::SkillsWatcher;
-use crate::skills_watcher::SkillsWatcherEvent;
 use crate::state::ActiveTurn;
 use crate::state::MailboxDeliveryPhase;
 use crate::state::PendingRequestPermissions;
@@ -393,7 +392,6 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) skills_manager: Arc<SkillsManager>,
     pub(crate) plugins_manager: Arc<PluginsManager>,
     pub(crate) mcp_manager: Arc<McpManager>,
-    pub(crate) skills_watcher: Arc<SkillsWatcher>,
     pub(crate) conversation_history: InitialHistory,
     pub(crate) session_source: SessionSource,
     pub(crate) thread_source: Option<ThreadSource>,
@@ -457,7 +455,6 @@ impl Codex {
             skills_manager,
             plugins_manager,
             mcp_manager,
-            skills_watcher,
             conversation_history,
             session_source,
             thread_source,
@@ -653,7 +650,6 @@ impl Codex {
             skills_manager,
             plugins_manager,
             mcp_manager.clone(),
-            skills_watcher,
             agent_control,
             environment_manager,
             analytics_events_client,
@@ -788,6 +784,11 @@ impl Codex {
         state.session_configuration.thread_config_snapshot()
     }
 
+    pub(crate) async fn thread_environment_selections(&self) -> Vec<TurnEnvironmentSelection> {
+        let state = self.session.state.lock().await;
+        state.session_configuration.environments.clone()
+    }
+
     pub(crate) fn state_db(&self) -> Option<state_db::StateDbHandle> {
         self.session.state_db()
     }
@@ -1021,29 +1022,6 @@ impl Session {
         self.out_of_band_elicitation_paused.send_replace(paused);
     }
 
-    fn start_skills_watcher_listener(self: &Arc<Self>) {
-        let mut rx = self.services.skills_watcher.subscribe();
-        let weak_sess = Arc::downgrade(self);
-        tokio::spawn(async move {
-            loop {
-                match rx.recv().await {
-                    Ok(SkillsWatcherEvent::SkillsChanged { .. }) => {
-                        let Some(sess) = weak_sess.upgrade() else {
-                            break;
-                        };
-                        let event = Event {
-                            id: sess.next_internal_sub_id(),
-                            msg: EventMsg::SkillsUpdateAvailable,
-                        };
-                        sess.send_event_raw(event).await;
-                    }
-                    Err(tokio::sync::broadcast::error::RecvError::Closed) => break,
-                    Err(tokio::sync::broadcast::error::RecvError::Lagged(_)) => continue,
-                }
-            }
-        });
-    }
-
     pub(crate) fn get_tx_event(&self) -> Sender<Event> {
         self.tx_event.clone()
     }