feat: 新增 YouTube/Bilibili/XHS 搜索 CLI 命令 + 修复 B站搜索

- cli.py: 新增 search-youtube / search-bilibili / search-xhs 子命令
- core.py: 新增 search_youtube / search_bilibili / search_xhs 方法
- bilibili.py: 搜索策略改为 yt-dlp → Exa fallback（服务器 IP 被 B站 412）
  - 本地机器走 yt-dlp bilisearch（有 title/uploader）
  - 服务器自动 fallback 到 Exa site:bilibili.com
  - 修复 mcporter 输出解析（Title:/URL: 格式）

测试结果: 15/15 全通（8 read + 7 search）

agent_reach/core.py

@@ -83,6 +83,24 @@ class AgentReach:
         results = await ch.search(query, config=self.config, limit=limit)
         return [r.to_dict() for r in results]
 
+    async def search_youtube(self, query: str, limit: int = 5) -> List[Dict[str, Any]]:
+        """Search YouTube via yt-dlp."""
+        ch = get_channel("youtube")
+        results = await ch.search(query, config=self.config, limit=limit)
+        return [r.to_dict() for r in results]
+
+    async def search_bilibili(self, query: str, limit: int = 5) -> List[Dict[str, Any]]:
+        """Search Bilibili. Tries yt-dlp first, falls back to Exa."""
+        ch = get_channel("bilibili")
+        results = await ch.search(query, config=self.config, limit=limit)
+        return [r.to_dict() for r in results]
+
+    async def search_xhs(self, query: str, limit: int = 10) -> List[Dict[str, Any]]:
+        """Search XiaoHongShu via mcporter."""
+        ch = get_channel("xiaohongshu")
+        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]: