feat(channels): add Discord channel (Exa search + public Invite API) (#230)

Zero-config channel (tier=0). Uses Discord Invite API to read public
server info (no auth), and Exa via mcporter for content search.
Tested: discord.gg/python returns 419K members, description, channel.
104 tests passing.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

agent_reach/channels/__init__.py

@@ -24,6 +24,7 @@ from .xiaoyuzhou import XiaoyuzhouChannel
 from .v2ex import V2EXChannel
 from .xueqiu import XueqiuChannel
 from .toutiao import ToutiaoChannel
+from .discord import DiscordChannel
 
 
 ALL_CHANNELS: List[Channel] = [
@@ -41,6 +42,7 @@ ALL_CHANNELS: List[Channel] = [
     V2EXChannel(),
     XueqiuChannel(),
     ToutiaoChannel(),
+    DiscordChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),