Allow extensions to skip host skill discovery (#40631)

## What changed

- Add the under-development `skip_host_skill_discovery` feature to bypass host
  skill snapshots during session warmup and turn setup.
- Let skill invocation contributors declare whether they require host-owned
  skills. Preserve host discovery when no contributors are registered or any
  contributor requires it.
- Keep executor and orchestrator skill catalogs and instruction loading
  available when host discovery is skipped.

## Testing

- Cover executor-only, orchestrator, mixed-contributor, and legacy host-skill
  behavior.

GitOrigin-RevId: 48169403090b234e1a304c6523633fd867df454a

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

@@ -25,6 +25,7 @@ use codex_protocol::protocol::ThreadHistoryMode;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_sandboxing::policy_transforms::effective_permission_profile;
 use codex_skills_extension::HostSkillsSnapshot;
+use codex_skills_extension::SkillLoadOutcome;
 use codex_utils_path_uri::PathUri;
 use futures::FutureExt;
 use futures::future::BoxFuture;
@@ -914,20 +915,29 @@ impl Session {
             &plugin_outcome,
             per_turn_config.codex_home.as_path(),
         );
-        let effective_skill_roots = plugin_outcome.effective_plugin_skill_roots();
-        let plugin_skill_snapshots = self
-            .services
-            .plugins_manager
-            .plugin_skill_snapshots_for_config(&plugins_input);
-        let skills_input = skills_load_input_from_config(&per_turn_config, effective_skill_roots)
-            .with_plugin_skill_snapshots(plugin_skill_snapshots);
-        let fs = primary_turn_environment
-            .map(|turn_environment| turn_environment.environment.get_filesystem());
-        let skills_snapshot = self
-            .services
-            .skills_service
-            .snapshot_for_config(&skills_input, fs)
-            .await;
+        let skills_snapshot = if per_turn_config
+            .features
+            .enabled(Feature::SkipHostSkillDiscovery)
+            && !self.services.extensions.requires_host_skill_discovery()
+        {
+            // Executor and orchestrator catalogs are supplied independently of host skills.
+            HostSkillsSnapshot::new(Arc::new(SkillLoadOutcome::default()))
+        } else {
+            let effective_skill_roots = plugin_outcome.effective_plugin_skill_roots();
+            let plugin_skill_snapshots = self
+                .services
+                .plugins_manager
+                .plugin_skill_snapshots_for_config(&plugins_input);
+            let skills_input =
+                skills_load_input_from_config(&per_turn_config, effective_skill_roots)
+                    .with_plugin_skill_snapshots(plugin_skill_snapshots);
+            let fs = primary_turn_environment
+                .map(|turn_environment| turn_environment.environment.get_filesystem());
+            self.services
+                .skills_service
+                .snapshot_for_config(&skills_input, fs)
+                .await
+        };
         let mut turn_context: TurnContext = Self::make_turn_context(
             self.thread_id(),
             self.session_id(),