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(_)

codex-rs/mcp-server/src/outgoing_message.rs

@@ -266,7 +266,6 @@ mod tests {
                 history_log_id: 1,
                 history_entry_count: 1000,
                 initial_messages: None,
-                skill_load_outcome: None,
                 rollout_path: rollout_file.path().to_path_buf(),
             }),
         };
@@ -306,7 +305,6 @@ mod tests {
             history_log_id: 1,
             history_entry_count: 1000,
             initial_messages: None,
-            skill_load_outcome: None,
             rollout_path: rollout_file.path().to_path_buf(),
         };
         let event = Event {