feat: 新增 Instagram、LinkedIn、Boss直聘 三个渠道

新增渠道：
- Instagram: 基于 instaloader (⭐9.8K)，读取帖子/Profile，Cookie 登录
- LinkedIn: 基于 linkedin-scraper-mcp (⭐900+) MCP 服务，Jina Reader fallback
- Boss直聘: 基于 mcp-bosszp MCP 服务，Jina Reader fallback

代码改动：
- 新建 channels/instagram.py, linkedin.py, bosszhipin.py
- 注册到 channels/__init__.py
- cli.py 添加 search-instagram/linkedin/bosszhipin 子命令
- cli.py 安装逻辑添加 instaloader 自动安装
- core.py 添加 search_instagram/linkedin/bosszhipin 方法
- README.md + docs/README_en.md 更新平台表格和选型表格
- docs/install.md 添加三个新渠道的配置说明和 Quick Reference

agent_reach/core.py

@@ -101,6 +101,24 @@ class AgentReach:
         results = await ch.search(query, config=self.config, limit=limit)
         return [r.to_dict() for r in results]
 
+    async def search_instagram(self, query: str, limit: int = 10) -> List[Dict[str, Any]]:
+        """Search Instagram via Exa."""
+        ch = get_channel("instagram")
+        results = await ch.search(query, config=self.config, limit=limit)
+        return [r.to_dict() for r in results]
+
+    async def search_linkedin(self, query: str, limit: int = 10) -> List[Dict[str, Any]]:
+        """Search LinkedIn via MCP or Exa."""
+        ch = get_channel("linkedin")
+        results = await ch.search(query, config=self.config, limit=limit)
+        return [r.to_dict() for r in results]
+
+    async def search_bosszhipin(self, query: str, limit: int = 10) -> List[Dict[str, Any]]:
+        """Search Boss直聘 via MCP or Exa."""
+        ch = get_channel("bosszhipin")
+        results = await ch.search(query, config=self.config, limit=limit)
+        return [r.to_dict() for r in results]
+
     # ── Health ──────────────────────────────────────────
 
     def doctor(self) -> Dict[str, dict]: