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

pyproject.toml

@@ -10,7 +10,7 @@ keywords = [
     "ai-agent", "llm-tools", "agent-infrastructure", "mcp",
     "web-reader", "web-scraper", "search",
     "twitter-scraper", "reddit-scraper", "youtube-transcript",
-    "bilibili", "xiaohongshu", "instagram",
+    "bilibili", "xiaohongshu",
     "ai-search", "cli", "automation",
     "claude-code", "cursor", "openai",
     "free-api", "no-api-key",