Reimplement skills loading using SkillsManager + skills/list op. (#7914)

refactor the way we load and manage skills:
1. Move skill discovery/caching into SkillsManager and reuse it across
sessions.
2. Add the skills/list API (Op::ListSkills/SkillsListResponse) to fetch
skills for one or more cwds. Also update app-server for VSCE/App;
3. Trigger skills/list during session startup so UIs preload skills and
handle errors immediately.

codex-rs/core/src/codex_delegate.rs

@@ -49,6 +49,7 @@ pub(crate) async fn run_codex_conversation_interactive(
         config,
         auth_manager,
         models_manager,
+        Arc::clone(&parent_session.services.skills_manager),
         initial_history.unwrap_or(InitialHistory::New),
         SessionSource::SubAgent(SubAgentSource::Review),
     )