fix(chat): query expert skills via symlinks and use $ prefix for Codex

Expert skills in the chat session were derived by intersecting built-in
experts with ACP availableCommands, which caused Codex experts to never
appear since Codex does not advertise skills through ACP.

- Add `experts_list_for_agent` backend API that checks symlink status
  across all global skill dirs for the given agent type
- Replace availableCommands-based expert filtering with symlink-based
  query, making the settings page the single source of truth
- Use `$` prefix for Codex expert skills while keeping `/` for slash
  commands and other agents' experts
- Disable the expert button when no experts are linked for the agent
- Invalidate per-agent expert cache after link/unlink in settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/experts.rs

@@ -682,6 +682,47 @@ pub async fn experts_list() -> Result<Vec<ExpertListItem>, ExpertsError> {
     Ok(out)
 }
 
+#[cfg_attr(feature = "tauri-runtime", tauri::command)]
+pub async fn experts_list_for_agent(
+    agent_type: AgentType,
+) -> Result<Vec<ExpertListItem>, ExpertsError> {
+    let _ = skill_storage_spec(agent_type)
+        .ok_or(ExpertsError::UnsupportedAgent(agent_type))?;
+
+    let dirs = scoped_skill_dirs(agent_type, AgentSkillScope::Global, None)
+        .map_err(|_| ExpertsError::UnsupportedAgent(agent_type))?;
+
+    let meta_list = bundled_metadata().to_vec();
+    let manifest = load_manifest();
+    let mut out = Vec::new();
+
+    for meta in meta_list {
+        let central_path = expert_central_path(&meta.id);
+        let is_linked = dirs.iter().any(|dir| {
+            let candidate = dir.join(&meta.id);
+            classify_link(&candidate, &central_path) == ExpertLinkState::LinkedToCodeg
+        });
+        if !is_linked {
+            continue;
+        }
+
+        let installed_centrally = central_path.exists();
+        let user_modified = manifest
+            .experts
+            .get(&meta.id)
+            .map(|e| e.pending_user_review)
+            .unwrap_or(false);
+
+        out.push(ExpertListItem {
+            metadata: meta,
+            installed_centrally,
+            user_modified,
+            central_path: central_path.to_string_lossy().to_string(),
+        });
+    }
+    Ok(out)
+}
+
 #[cfg_attr(feature = "tauri-runtime", tauri::command)]
 pub async fn experts_get_install_status(
     expert_id: String,