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/core/tests/common/test_codex.rs

@@ -107,8 +107,11 @@ impl TestCodexBuilder {
         let (config, cwd) = self.prepare_config(server, &home).await?;
 
         let auth = self.auth.clone();
-        let conversation_manager =
-            ConversationManager::with_models_provider(auth.clone(), config.model_provider.clone());
+        let conversation_manager = ConversationManager::with_models_provider_and_home(
+            auth.clone(),
+            config.model_provider.clone(),
+            config.codex_home.clone(),
+        );
 
         let new_conversation = match resume_from {
             Some(path) => {