refactor: remove Douyin, Weibo and WeChat channels (#347)

All three had rotted past honest usability:
- Douyin's upstream (yzfly/douyin-mcp-server) is archived and required a
  4-step manual local-server setup nobody could complete
- Weibo depended on an unmaintained personal fork (mcp-server-weibo)
- WeChat full-article reading was increasingly blocked by anti-bot
  (#339) while doctor still advertised it as zero-config

Removes the channel files, installers, skill routing/trigger entries,
reference sections and README rows (zh+en). Honest counts: 13 platforms,
6 zero-config. They can return when maintained upstreams exist.

Follows the v1.4.0 precedent of removing Discord/Toutiao (#234).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent_reach/channels/__init__.py

@@ -16,10 +16,7 @@ from .rss import RSSChannel
 from .bilibili import BilibiliChannel
 from .exa_search import ExaSearchChannel
 from .xiaohongshu import XiaoHongShuChannel
-from .douyin import DouyinChannel
 from .linkedin import LinkedInChannel
-from .wechat import WeChatChannel
-from .weibo import WeiboChannel
 from .xiaoyuzhou import XiaoyuzhouChannel
 from .v2ex import V2EXChannel
 from .xueqiu import XueqiuChannel
@@ -32,10 +29,7 @@ ALL_CHANNELS: List[Channel] = [
     RedditChannel(),
     BilibiliChannel(),
     XiaoHongShuChannel(),
-    DouyinChannel(),
     LinkedInChannel(),
-    WeChatChannel(),
-    WeiboChannel(),
     XiaoyuzhouChannel(),
     V2EXChannel(),
     XueqiuChannel(),