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/src/event_processor_with_human_output.rs

@@ -572,6 +572,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             | EventMsg::GetHistoryEntryResponse(_)
             | EventMsg::McpListToolsResponse(_)
             | EventMsg::ListCustomPromptsResponse(_)
+            | EventMsg::ListSkillsResponse(_)
             | EventMsg::RawResponseItem(_)
             | EventMsg::UserMessage(_)
             | EventMsg::EnteredReviewMode(_)