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/tui2/src/chatwidget/tests.rs

@@ -125,7 +125,6 @@ fn resumed_initial_messages_render_history() {
                 message: "assistant reply".to_string(),
             }),
         ]),
-        skill_load_outcome: None,
         rollout_path: rollout_file.path().to_path_buf(),
     };