refactor: remove Discord and Toutiao channels (#234)

Discord only returns server metadata (no messages without Bot Token),
not useful enough. Toutiao search was fragile (HTML scraping). Both
can be revisited when better upstream tools appear.

75 tests passing.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent_reach/channels/__init__.py

@@ -23,8 +23,6 @@ from .weibo import WeiboChannel
 from .xiaoyuzhou import XiaoyuzhouChannel
 from .v2ex import V2EXChannel
 from .xueqiu import XueqiuChannel
-from .toutiao import ToutiaoChannel
-from .discord import DiscordChannel
 
 
 ALL_CHANNELS: List[Channel] = [
@@ -41,8 +39,6 @@ ALL_CHANNELS: List[Channel] = [
     XiaoyuzhouChannel(),
     V2EXChannel(),
     XueqiuChannel(),
-    ToutiaoChannel(),
-    DiscordChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),