fix(chat): use ~/.codex/skills/ for Codex expert symlinks and keep expert button always enabled

Change Codex skill storage to use only ~/.codex/skills/ instead of
~/.agents/skills/, and never disable the expert skills button so users
can always access the empty-state hint. Also fix stale expert list in
conversation window after linking in the separate settings window by
re-fetching on window focus.

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

src/components/chat/message-input.tsx

@@ -1757,7 +1757,7 @@ export function MessageInput({
             <DropdownMenu>
               <DropdownMenuTrigger asChild>
                 <Button
-                  disabled={disabled || availableExperts.length === 0}
+                  disabled={disabled}
                   variant="outline"
                   size="icon"
                   className="h-6 w-6 shrink-0 bg-transparent"