feat(brainstorm): enforce Chinese language for all user questions

Add mandatory Chinese language rule for AskUserQuestion in artifacts.md and synthesis.md:
- Phase 3 Rules: Questions MUST be asked in Chinese (用中文提问)
- Question Generation Guidelines: ALL questions MUST be in Chinese (所有问题必须用中文)
- Phase 4 (synthesis.md): ALL AskUserQuestion calls MUST use Chinese

Rationale: Improve user understanding and interaction experience for Chinese-speaking users

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude/commands/workflow/brainstorm/synthesis.md

@@ -153,6 +153,8 @@ Return JSON array:
 
 **Main flow handles all user interaction**:
 
+**⚠️ CRITICAL**: ALL AskUserQuestion calls MUST use Chinese (所有问题必须用中文) for better user understanding
+
 1. **Present Enhancement Options**:
 ```python
 AskUserQuestion(
@@ -170,6 +172,7 @@ AskUserQuestion(
 ```
 
 2. **Generate Clarification Questions** (based on analysis agent output):
+   - ✅ **ALL questions MUST be in Chinese (所有问题必须用中文)**
    - Use 9-category taxonomy scan results
    - Create max 5 prioritized questions
    - Each with 2-4 options + descriptions