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/mcp-server/src/codex_tool_runner.rs

@@ -279,6 +279,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::McpToolCallEnd(_)
                     | EventMsg::McpListToolsResponse(_)
                     | EventMsg::ListCustomPromptsResponse(_)
+                    | EventMsg::ListSkillsResponse(_)
                     | EventMsg::ExecCommandBegin(_)
                     | EventMsg::TerminalInteraction(_)
                     | EventMsg::ExecCommandOutputDelta(_)