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/chat/conversation-shell.tsx

@@ -1,5 +1,6 @@
 import type { ReactNode } from "react"
 import type {
+  AgentType,
   ConnectionStatus,
   PromptCapabilitiesInfo,
   PromptDraft,
@@ -38,6 +39,7 @@ interface ConversationShellProps {
   selectedModeId?: string | null
   onModeChange?: (modeId: string) => void
   onConfigOptionChange?: (configId: string, valueId: string) => void
+  agentType?: AgentType | null
   availableCommands?: AvailableCommandInfo[] | null
   attachmentTabId?: string | null
   draftStorageKey?: string | null
@@ -78,6 +80,7 @@ export function ConversationShell({
   selectedModeId,
   onModeChange,
   onConfigOptionChange,
+  agentType,
   availableCommands,
   attachmentTabId,
   draftStorageKey,
@@ -123,6 +126,7 @@ export function ConversationShell({
           selectedModeId={selectedModeId}
           onModeChange={onModeChange}
           onConfigOptionChange={onConfigOptionChange}
+          agentType={agentType}
           availableCommands={availableCommands}
           attachmentTabId={attachmentTabId}
           draftStorageKey={draftStorageKey}