fix(gateway): wire Telegram plugin into gateway plugin host

Add @mosaic/telegram-plugin dependency to gateway, implement
TelegramChannelPluginAdapter, replace stub warning with real plugin
instantiation, and document all Phase 5 env vars in .env.example.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/package.json

@@ -20,6 +20,7 @@
     "@mosaic/coord": "workspace:^",
     "@mosaic/db": "workspace:^",
     "@mosaic/discord-plugin": "workspace:^",
+    "@mosaic/telegram-plugin": "workspace:^",
     "@mosaic/log": "workspace:^",
     "@mosaic/memory": "workspace:^",
     "@mosaic/types": "workspace:^",