remove(instagram): 移除 Instagram 渠道

Instagram 反爬封杀导致所有开源工具（instaloader 等）失效，
无论有无 cookies 都无法正常使用。

- 删除 instagram.py 渠道文件
- 移除 CLI 中 search-instagram、configure instagram-cookies 等命令
- 移除 setup/doctor 中 instaloader 依赖检查
- 更新 README、docs、SKILL.md、pyproject.toml

上游 issue: instaloader#2585, instaloader#2648
Relates to: #13

agent_reach/core.py

@@ -101,12 +101,6 @@ 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")