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/features/src/lib.rs

@@ -209,6 +209,8 @@ pub enum Feature {
     Plugins,
     /// Discover selected-root plugin and skill manifests through one high-level exec-server RPC.
     ExecutorCapabilityDiscovery,
+    /// Skip host skill snapshots when no registered contributor requires them.
+    SkipHostSkillDiscovery,
     /// Removed compatibility flag for plugin-bundled lifecycle hooks.
     PluginHooks,
     /// Allow the in-app browser pane in desktop apps.
@@ -1249,6 +1251,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::SkipHostSkillDiscovery,
+        key: "skip_host_skill_discovery",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::PluginHooks,
         key: "plugin_hooks",