docs(chat): consolidate multi-role protocol

src/app/chat/chat-screen.tsx

@@ -98,7 +98,7 @@ export function ChatScreen() {
     [imageMessageId, visibleMessages],
   );
 
-  // isGuest 派生（chat-screen 是唯一知道这个值的地方 —— chat 机器无 isGuest 字段）
+  // Guest status belongs to auth state and is not duplicated in the Chat actor.
   const isGuest = deriveIsGuest(authState.loginStatus);
 
   // 发送能力由后端统一返回，当前只处理积分不足引导。
@@ -251,7 +251,6 @@ export function ChatScreen() {
         </div>
 
         <div className="relative z-1 flex min-h-0 flex-[1_1_auto] flex-col">
-          {/* isGuest 派生自 auth.loginStatus */}
           <ChatHeader
             isGuest={isGuest}
             showBrowserHint={shouldShowBrowserHint}