fix: uncomment Discord import in channel barrel

Discord was directly imported in src/index.ts before the barrel wiring.
Moving to the barrel without uncommenting it broke Discord.

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

src/channels/index.ts

@@ -2,7 +2,7 @@
 // Each import triggers the channel module's registerChannelAdapter() call.
 
 // discord
-// import './discord.js';
+import './discord.js';
 
 // slack
 // import './slack.js';