feat: add structured questions (AskUserQuestion) to assistant chat

Add interactive multiple-choice question support to the project assistant,
allowing it to present clickable options when clarification is needed.

Backend changes:
- Add ask_user MCP tool to feature_mcp.py with input validation
- Add mcp__features__ask_user to assistant allowed tools list
- Intercept ask_user tool calls in _query_claude() to yield question messages
- Add answer WebSocket message handler in assistant_chat router
- Document ask_user tool in assistant system prompt

Frontend changes:
- Add AssistantChatQuestionMessage type and update server message union
- Add currentQuestions state and sendAnswer() to useAssistantChat hook
- Handle question WebSocket messages by attaching to last assistant message
- Render QuestionOptions component between messages and input area
- Disable text input while structured questions are active

Flow: Claude calls ask_user → backend intercepts → WebSocket question message →
frontend renders QuestionOptions → user clicks options → answer sent back →
Claude receives formatted answer and continues conversation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/AssistantChat.tsx

@@ -11,6 +11,7 @@ import { Send, Loader2, Wifi, WifiOff, Plus, History } from 'lucide-react'
 import { useAssistantChat } from '../hooks/useAssistantChat'
 import { ChatMessage as ChatMessageComponent } from './ChatMessage'
 import { ConversationHistory } from './ConversationHistory'
+import { QuestionOptions } from './QuestionOptions'
 import type { ChatMessage } from '../lib/types'
 import { isSubmitEnter } from '../lib/keyboard'
 import { Button } from '@/components/ui/button'
@@ -52,8 +53,10 @@ export function AssistantChat({
     isLoading,
     connectionStatus,
     conversationId: activeConversationId,
+    currentQuestions,
     start,
     sendMessage,
+    sendAnswer,
     clearMessages,
   } = useAssistantChat({
     projectName,
@@ -268,6 +271,16 @@ export function AssistantChat({
         </div>
       )}
 
+      {/* Structured questions from assistant */}
+      {currentQuestions && (
+        <div className="border-t border-border bg-background">
+          <QuestionOptions
+            questions={currentQuestions}
+            onSubmit={sendAnswer}
+          />
+        </div>
+      )}
+
       {/* Input area */}
       <div className="border-t border-border p-4 bg-card">
         <div className="flex gap-2">
@@ -277,13 +290,13 @@ export function AssistantChat({
             onChange={(e) => setInputValue(e.target.value)}
             onKeyDown={handleKeyDown}
             placeholder="Ask about the codebase..."
-            disabled={isLoading || isLoadingConversation || connectionStatus !== 'connected'}
+            disabled={isLoading || isLoadingConversation || connectionStatus !== 'connected' || !!currentQuestions}
             className="flex-1 resize-none min-h-[44px] max-h-[120px]"
             rows={1}
           />
           <Button
             onClick={handleSend}
-            disabled={!inputValue.trim() || isLoading || isLoadingConversation || connectionStatus !== 'connected'}
+            disabled={!inputValue.trim() || isLoading || isLoadingConversation || connectionStatus !== 'connected' || !!currentQuestions}
             title="Send message"
           >
             {isLoading ? (
@@ -294,7 +307,7 @@ export function AssistantChat({
           </Button>
         </div>
         <p className="text-xs text-muted-foreground mt-2">
-          Press Enter to send, Shift+Enter for new line
+          {currentQuestions ? 'Select an option above to continue' : 'Press Enter to send, Shift+Enter for new line'}
         </p>
       </div>
     </div>