feat: add WeChat Official Account channel (#54) (#67)

New channel: wechat (微信公众号文章)
- Backend: wechat-article-for-ai (Camoufox stealth browser)
- Can read mp.weixin.qq.com article URLs → clean Markdown
- Bypasses WeChat anti-bot detection via stealth Firefox
- Doctor checks for camoufox package availability
- SKILL.md updated with usage examples

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

agent_reach/channels/__init__.py

@@ -19,6 +19,7 @@ from .xiaohongshu import XiaoHongShuChannel
 from .douyin import DouyinChannel
 from .linkedin import LinkedInChannel
 from .bosszhipin import BossZhipinChannel
+from .wechat import WeChatChannel
 
 
 # Channel registry
@@ -32,6 +33,7 @@ ALL_CHANNELS: List[Channel] = [
     DouyinChannel(),
     LinkedInChannel(),
     BossZhipinChannel(),
+    WeChatChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),