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/app-server-protocol/src/protocol/common.rs

@@ -121,6 +121,10 @@ client_request_definitions! {
         params: v2::ThreadCompactParams,
         response: v2::ThreadCompactResponse,
     },
+    SkillsList => "skills/list" {
+        params: v2::SkillsListParams,
+        response: v2::SkillsListResponse,
+    },
     TurnStart => "turn/start" {
         params: v2::TurnStartParams,
         response: v2::TurnStartResponse,