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>

server/services/assistant_chat_session.py

@@ -47,8 +47,13 @@ FEATURE_MANAGEMENT_TOOLS = [
     "mcp__features__feature_skip",
 ]
 
+# Interactive tools
+INTERACTIVE_TOOLS = [
+    "mcp__features__ask_user",
+]
+
 # Combined list for assistant
-ASSISTANT_FEATURE_TOOLS = READONLY_FEATURE_MCP_TOOLS + FEATURE_MANAGEMENT_TOOLS
+ASSISTANT_FEATURE_TOOLS = READONLY_FEATURE_MCP_TOOLS + FEATURE_MANAGEMENT_TOOLS + INTERACTIVE_TOOLS
 
 # Read-only built-in tools (no Write, Edit, Bash)
 READONLY_BUILTIN_TOOLS = [
@@ -123,6 +128,9 @@ If the user asks you to modify code, explain that you're a project assistant and
 - **feature_create_bulk**: Create multiple features at once
 - **feature_skip**: Move a feature to the end of the queue
 
+**Interactive:**
+- **ask_user**: Present structured multiple-choice questions to the user. Use this when you need to clarify requirements, offer design choices, or guide a decision. The user sees clickable option buttons and their selection is returned as your next message.
+
 ## Creating Features
 
 When a user asks to add a feature, use the `feature_create` or `feature_create_bulk` MCP tools directly:
@@ -402,6 +410,17 @@ class AssistantChatSession:
                     elif block_type == "ToolUseBlock" and hasattr(block, "name"):
                         tool_name = block.name
                         tool_input = getattr(block, "input", {})
+
+                        # Intercept ask_user tool calls -> yield as question message
+                        if tool_name == "mcp__features__ask_user":
+                            questions = tool_input.get("questions", [])
+                            if questions:
+                                yield {
+                                    "type": "question",
+                                    "questions": questions,
+                                }
+                                continue
+
                         yield {
                             "type": "tool_call",
                             "tool": tool_name,