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

docs/README_en.md

@@ -58,6 +58,9 @@ Copy that to your Agent. A few minutes later, it can read tweets, search Reddit,
 | 🌐 **Web** | Read | Zero config | Any URL → clean Markdown ([Jina Reader](https://github.com/jina-ai/reader) ⭐9.8K) |
 | 🐦 **Twitter/X** | Read · Search | Zero config / Cookie | Single tweets readable out of the box. Cookie unlocks search, timeline, posting ([bird](https://github.com/steipete/bird)) |
 | 📕 **XiaoHongShu** | Read · Search · **Post · Comment · Like** | mcporter | Via [xiaohongshu-mcp](https://github.com/user/xiaohongshu-mcp) internal API, install and go |
+| 📷 **Instagram** | Read · Search | instaloader | Posts, profiles, hashtags ([instaloader](https://github.com/instaloader/instaloader) ⭐9.8K) |
+| 💼 **LinkedIn** | Read · Search | mcporter / Jina | Profiles, companies, job search ([linkedin-scraper-mcp](https://github.com/stickerdaniel/linkedin-mcp-server) ⭐900+) |
+| 🏢 **Boss直聘** | Read · Search | mcporter / Jina | Job search, greet recruiters ([mcp-bosszp](https://github.com/mucsbr/mcp-bosszp)) |
 | 🔍 **Web Search** | Search | Auto-configured | Auto-configured during install, free, no API key ([Exa](https://exa.ai) via [mcporter](https://github.com/nicepkg/mcporter)) |
 | 📦 **GitHub** | Read · Search | Zero config | [gh CLI](https://cli.github.com) powered. Public repos work immediately. `gh auth login` unlocks Fork, Issue, PR |
 | 📺 **YouTube** | Read · **Search** | Zero config | Subtitles + search across 1800+ video sites ([yt-dlp](https://github.com/yt-dlp/yt-dlp) ⭐148K) |
@@ -182,6 +185,9 @@ channels/
 ├── bilibili.py     → yt-dlp           ← swap to bilibili-api…
 ├── reddit.py       → JSON API + Exa  ← swap to PRAW, Pushshift…
 ├── xiaohongshu.py  → mcporter MCP    ← swap to other XHS tools…
+├── instagram.py    → instaloader     ← swap to instagrapi, official API…
+├── linkedin.py     → linkedin-mcp    ← swap to LinkedIn API…
+├── bosszhipin.py   → mcp-bosszp      ← swap to other job tools…
 ├── rss.py          → feedparser       ← swap to atoma…
 ├── exa_search.py   → mcporter MCP    ← swap to Tavily, SerpAPI…
 └── __init__.py     → Channel registry
@@ -198,6 +204,9 @@ channels/
 | GitHub | [gh CLI](https://cli.github.com) | Official tool, full API after auth |
 | Read RSS | [feedparser](https://github.com/kurtmckee/feedparser) | Python ecosystem standard, 2.3K stars |
 | XiaoHongShu | [xiaohongshu-mcp](https://github.com/user/xiaohongshu-mcp) | Internal API, bypasses anti-bot |
+| Instagram | [instaloader](https://github.com/instaloader/instaloader) | 9.8K stars, Python CLI, cookie auth, free |
+| LinkedIn | [linkedin-scraper-mcp](https://github.com/stickerdaniel/linkedin-mcp-server) | 900+ stars, MCP server, browser automation |
+| Boss直聘 | [mcp-bosszp](https://github.com/mucsbr/mcp-bosszp) | MCP server, job search + recruiter greeting |
 
 > 📌 These are the *current* choices. Don't like one? Swap out the file. That's the whole point of scaffolding.