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/web/handlers/experts.rs

@@ -12,6 +12,12 @@ pub struct ExpertIdParams {
     pub expert_id: String,
 }
 
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct AgentTypeOnlyParams {
+    pub agent_type: AgentType,
+}
+
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ExpertAgentParams {
@@ -26,6 +32,15 @@ pub async fn experts_list() -> Result<Json<Vec<ExpertListItem>>, AppCommandError
     Ok(Json(result))
 }
 
+pub async fn experts_list_for_agent(
+    Json(params): Json<AgentTypeOnlyParams>,
+) -> Result<Json<Vec<ExpertListItem>>, AppCommandError> {
+    let result = experts_commands::experts_list_for_agent(params.agent_type)
+        .await
+        .map_err(|e| AppCommandError::task_execution_failed(e.to_string()))?;
+    Ok(Json(result))
+}
+
 pub async fn experts_get_install_status(
     Json(params): Json<ExpertIdParams>,
 ) -> Result<Json<Vec<ExpertInstallStatus>>, AppCommandError> {