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/ext/skills/src/extension.rs

@@ -324,6 +324,10 @@ impl<C> SkillInvocationContributor for SkillsExtension<C>
 where
     C: Send + Sync + 'static,
 {
+    fn requires_host_skill_discovery(&self) -> bool {
+        self.providers.has_host_provider()
+    }
+
     fn on_skill_invocation<'a>(
         &'a self,
         input: SkillInvocationInput<'a>,