feat(skill): refactor skill system with router + references structure (#192)

Refactors SKILL.md into a router + reference documentation system:
- Main SKILL.md now contains routing table and quick commands
- Detailed docs split into references/ (search, social, career, dev, web, video)
- CLI skill install copies entire directory including references/
- Resolves conflicts with recent merges (#210, #213)
- Fixes xreach→bird in quick commands

Original PR by @Citrus086, merged with conflict resolution.

Co-Authored-By: Citrus086 <Citrus086@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent_reach/skill/references/search.md

@@ -0,0 +1,33 @@
+# 搜索工具
+
+Exa AI 搜索引擎。
+
+## Exa AI 搜索
+
+高质量 AI 搜索引擎，擅长技术和代码搜索。
+
+```bash
+mcporter call 'exa.web_search_exa(query: "query", numResults: 5)'
+mcporter call 'exa.get_code_context_exa(query: "code question", tokensNum: 3000)'
+```
+
+### 使用场景
+
+| 场景 | 参数 |
+|-----|------|
+| 网页搜索 | `web_search_exa(query: "...", numResults: 5)` |
+| 代码搜索 | `get_code_context_exa(query: "...", tokensNum: 3000)` |
+
+### 特点
+
+- 擅长英文内容和技术文档
+- 支持代码上下文搜索
+- 结果质量高
+
+## 与其他搜索工具对比
+
+| 工具 | 来源 | 适用场景 |
+|-----|------|---------|
+| Exa | agent-reach | 英文/技术/代码搜索 |
+| 智谱搜索 | my-mcp-tools | 中文搜索 |
+| GitHub 搜索 | agent-reach (dev.md) | 仓库/代码搜索 |