docs: clarify brainstorm vs plan workflow and add decision guide

Major improvements to workflow understanding and guidance:

1. New Document: WORKFLOW_DECISION_GUIDE.md
   - Comprehensive decision flowchart for full software lifecycle
   - Interactive Mermaid diagram with 40+ decision points
   - Clear guidance on when to use each command
   - Real-world scenarios for all workflow types
   - Quick reference tables by knowledge level, project stage, and mode
   - Expert tips and common pitfalls

2. Clarified Brainstorm vs Plan Relationship
   Core concept correction:
   - Brainstorm: Use when you know WHAT to build, NOT HOW
   - Plan: Use when you know both WHAT and HOW
   - Plan can accept user input OR brainstorm output

3. Updated Documentation Files
   - GETTING_STARTED.md: Added clear distinction and decision criteria
   - GETTING_STARTED_CN.md: Chinese version with same clarifications
   - FAQ.md: Enhanced brainstorm usage explanation with workflow comparison table
   - README.md: Added Workflow Decision Guide link
   - README_CN.md: Added Chinese link to decision guide

4. Key Improvements
   When to Use Brainstorming:
   - Unclear solution approach (multiple ways to solve)
   - Architectural exploration needed
   - Requirements clarification (high-level clear, details not)
   - Multiple trade-offs to analyze
   - Unfamiliar domain

   When to Skip Brainstorming:
   - Clear implementation approach
   - Similar to existing code patterns
   - Well-defined requirements
   - Simple features

5. Decision Flowchart Covers:
   - Ideation phase (know what to build?)
   - Design phase (know how to build?)
   - UI design phase (need UI design?)
   - Planning phase (complexity level?)
   - Testing phase (TDD, post-test, test-fix?)
   - Review phase (security, architecture, quality?)
   - Completion

Benefits:
- Eliminates confusion about brainstorm usage
- Provides clear decision criteria for all commands
- Visual flowchart helps users navigate workflows
- Comprehensive coverage of all development stages
- Reduces trial-and-error in workflow selection

Files modified: 5
Files added: 1 (WORKFLOW_DECISION_GUIDE.md)

GETTING_STARTED_CN.md

@@ -136,9 +136,16 @@
 - **综合规格说明**：生成整合的需求和设计文档
 
 **何时使用头脑风暴**：
-- 需要多视角分析的复杂功能
-- 具有重大影响的架构决策
-- 实现前需要详尽需求分析
+- **知道要做什么，但不知道怎么做** - 需要探索解决方案
+- **存在多个解决路径** - 需要专家分析选择最佳方案
+- **技术需求不明确** - 需要澄清架构、数据模型、API
+- **重大架构决策** - 需要多角度分析再做决定
+
+**何时跳过头脑风暴**（直接使用 `/workflow:plan`）：
+- 已经知道实现方法
+- 一开始就有清晰的技术需求
+- 简单直接的功能
+- 与代码库中现有实现类似
 
 ### 场景 4：质量保证 - 行动计划验证