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/components/conversations/conversation-detail-panel.tsx

@@ -895,6 +895,7 @@ const ConversationTabView = memo(function ConversationTabView({
       selectedModeId={selectedModeId}
       onModeChange={handleModeChange}
       onConfigOptionChange={handleSetConfigOption}
+      agentType={selectedAgent}
       availableCommands={connectionCommands}
       attachmentTabId={tabId}
       draftStorageKey={draftStorageKey}
@@ -966,6 +967,7 @@ const ConversationTabView = memo(function ConversationTabView({
               selectedModeId={selectedModeId}
               onModeChange={handleModeChange}
               onConfigOptionChange={handleSetConfigOption}
+              agentType={selectedAgent}
               availableCommands={connectionCommands}
               attachmentTabId={tabId}
               draftStorageKey={draftStorageKey}