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/exec/tests/event_processor_with_json_output.rs

@@ -85,7 +85,6 @@ fn session_configured_produces_thread_started_event() {
             history_log_id: 0,
             history_entry_count: 0,
             initial_messages: None,
-            skill_load_outcome: None,
             rollout_path,
         }),
     );