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/lib/api.ts

@@ -340,6 +340,12 @@ export async function expertsList(): Promise<ExpertListItem[]> {
   return getTransport().call("experts_list")
 }
 
+export async function expertsListForAgent(
+  agentType: AgentType
+): Promise<ExpertListItem[]> {
+  return getTransport().call("experts_list_for_agent", { agentType })
+}
+
 export async function expertsGetInstallStatus(
   expertId: string
 ): Promise<ExpertInstallStatus[]> {