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()
     }

codex-rs/core/src/session/session.rs

@@ -364,7 +364,6 @@ impl Session {
         skills_manager: Arc<SkillsManager>,
         plugins_manager: Arc<PluginsManager>,
         mcp_manager: Arc<McpManager>,
-        skills_watcher: Arc<SkillsWatcher>,
         agent_control: AgentControl,
         environment_manager: Arc<EnvironmentManager>,
         analytics_events_client: Option<AnalyticsEventsClient>,
@@ -846,7 +845,6 @@ impl Session {
                 skills_manager,
                 plugins_manager: Arc::clone(&plugins_manager),
                 mcp_manager: Arc::clone(&mcp_manager),
-                skills_watcher,
                 agent_control,
                 network_proxy,
                 network_approval: Arc::clone(&network_approval),
@@ -935,8 +933,6 @@ impl Session {
                 sess.send_event_raw(event).await;
             }
 
-            // Start the watcher after SessionConfigured so it cannot emit earlier events.
-            sess.start_skills_watcher_listener();
             let mut required_mcp_servers: Vec<String> = mcp_servers
                 .iter()
                 .filter(|(_, server)| server.enabled() && server.required())

codex-rs/core/src/session/tests.rs

@@ -3725,7 +3725,6 @@ async fn session_new_fails_when_zsh_fork_enabled_without_zsh_path() {
         skills_manager,
         plugins_manager,
         mcp_manager,
-        Arc::new(SkillsWatcher::noop()),
         AgentControl::default(),
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         /*analytics_events_client*/ None,
@@ -3837,7 +3836,6 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
             .expect("create environment"),
     );
 
-    let skills_watcher = Arc::new(SkillsWatcher::noop());
     let services = SessionServices {
         mcp_connection_manager: Arc::new(RwLock::new(McpConnectionManager::new_uninitialized(
             &config.permissions.approval_policy,
@@ -3873,7 +3871,6 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         skills_manager,
         plugins_manager,
         mcp_manager,
-        skills_watcher,
         agent_control,
         network_proxy: None,
         network_approval: Arc::clone(&network_approval),
@@ -4064,7 +4061,6 @@ async fn make_session_with_config_and_rx(
         skills_manager,
         plugins_manager,
         mcp_manager,
-        Arc::new(SkillsWatcher::noop()),
         AgentControl::default(),
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         /*analytics_events_client*/ None,
@@ -4167,7 +4163,6 @@ async fn make_session_with_history_source_and_agent_control_and_rx(
         skills_manager,
         plugins_manager,
         mcp_manager,
-        Arc::new(SkillsWatcher::noop()),
         agent_control,
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         /*analytics_events_client*/ None,
@@ -5556,7 +5551,6 @@ where
             .expect("create environment"),
     );
 
-    let skills_watcher = Arc::new(SkillsWatcher::noop());
     let services = SessionServices {
         mcp_connection_manager: Arc::new(RwLock::new(McpConnectionManager::new_uninitialized(
             &config.permissions.approval_policy,
@@ -5592,7 +5586,6 @@ where
         skills_manager,
         plugins_manager,
         mcp_manager,
-        skills_watcher,
         agent_control,
         network_proxy: None,
         network_approval: Arc::clone(&network_approval),

codex-rs/core/src/session/tests/guardian_tests.rs

@@ -728,7 +728,6 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         /*bundled_skills_enabled*/ true,
     ));
     let mcp_manager = Arc::new(McpManager::new(Arc::clone(&plugins_manager)));
-    let skills_watcher = Arc::new(SkillsWatcher::noop());
     let thread_store = Arc::new(codex_thread_store::LocalThreadStore::new(
         codex_thread_store::LocalThreadStoreConfig::from_config(&config),
         /*state_db*/ None,
@@ -743,7 +742,6 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         skills_manager,
         plugins_manager,
         mcp_manager,
-        skills_watcher,
         conversation_history: InitialHistory::New,
         session_source: SessionSource::SubAgent(SubAgentSource::Other(
             GUARDIAN_REVIEWER_NAME.to_string(),

codex-rs/core/src/session/turn.rs

@@ -1505,7 +1505,6 @@ pub(super) fn realtime_text_for_event(msg: &EventMsg) -> Option<String> {
         | EventMsg::StreamError(_)
         | EventMsg::TurnDiff(_)
         | EventMsg::RealtimeConversationListVoicesResponse(_)
-        | EventMsg::SkillsUpdateAvailable
         | EventMsg::PlanUpdate(_)
         | EventMsg::TurnAborted(_)
         | EventMsg::ShutdownComplete