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/src/codex_message_processor.rs

@@ -81,6 +81,8 @@ use codex_app_server_protocol::ServerNotification;
 use codex_app_server_protocol::SessionConfiguredNotification;
 use codex_app_server_protocol::SetDefaultModelParams;
 use codex_app_server_protocol::SetDefaultModelResponse;
+use codex_app_server_protocol::SkillsListParams;
+use codex_app_server_protocol::SkillsListResponse;
 use codex_app_server_protocol::Thread;
 use codex_app_server_protocol::ThreadArchiveParams;
 use codex_app_server_protocol::ThreadArchiveResponse;
@@ -373,6 +375,9 @@ impl CodexMessageProcessor {
                 self.send_unimplemented_error(request_id, "thread/compact")
                     .await;
             }
+            ClientRequest::SkillsList { request_id, params } => {
+                self.skills_list(request_id, params).await;
+            }
             ClientRequest::TurnStart { request_id, params } => {
                 self.turn_start(request_id, params).await;
             }
@@ -2615,6 +2620,42 @@ impl CodexMessageProcessor {
             .await;
     }
 
+    async fn skills_list(&self, request_id: RequestId, params: SkillsListParams) {
+        let SkillsListParams { cwds } = params;
+        let cwds = if cwds.is_empty() {
+            vec![self.config.cwd.clone()]
+        } else {
+            cwds
+        };
+
+        let data = if self.config.features.enabled(Feature::Skills) {
+            let skills_manager = self.conversation_manager.skills_manager();
+            cwds.into_iter()
+                .map(|cwd| {
+                    let outcome = skills_manager.skills_for_cwd(&cwd);
+                    let errors = errors_to_info(&outcome.errors);
+                    let skills = skills_to_info(&outcome.skills);
+                    codex_app_server_protocol::SkillsListEntry {
+                        cwd,
+                        skills,
+                        errors,
+                    }
+                })
+                .collect()
+        } else {
+            cwds.into_iter()
+                .map(|cwd| codex_app_server_protocol::SkillsListEntry {
+                    cwd,
+                    skills: Vec::new(),
+                    errors: Vec::new(),
+                })
+                .collect()
+        };
+        self.outgoing
+            .send_response(request_id, SkillsListResponse { data })
+            .await;
+    }
+
     async fn interrupt_conversation(
         &mut self,
         request_id: RequestId,
@@ -3260,6 +3301,32 @@ impl CodexMessageProcessor {
     }
 }
 
+fn skills_to_info(
+    skills: &[codex_core::skills::SkillMetadata],
+) -> Vec<codex_app_server_protocol::SkillMetadata> {
+    skills
+        .iter()
+        .map(|skill| codex_app_server_protocol::SkillMetadata {
+            name: skill.name.clone(),
+            description: skill.description.clone(),
+            path: skill.path.clone(),
+            scope: skill.scope.into(),
+        })
+        .collect()
+}
+
+fn errors_to_info(
+    errors: &[codex_core::skills::SkillError],
+) -> Vec<codex_app_server_protocol::SkillErrorInfo> {
+    errors
+        .iter()
+        .map(|err| codex_app_server_protocol::SkillErrorInfo {
+            path: err.path.clone(),
+            message: err.message.clone(),
+        })
+        .collect()
+}
+
 async fn derive_config_from_params(
     overrides: ConfigOverrides,
     cli_overrides: Option<std::collections::HashMap<String, serde_json::Value>>,