feat(channels): add Slack channel integration with Socket Mode

Add Slack as a messaging channel using Socket Mode (WebSocket), bringing
the total supported channels to 8. Features include bidirectional
messaging, thread support with per-thread session context, @mention
handling, ack reactions (👀/✅), slash commands,
file/attachment support with Groq Whisper audio transcription, and
allowlist filtering by Slack user ID.

Closes #31

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

config.example.json

@@ -43,6 +43,12 @@
       "client_id": "YOUR_CLIENT_ID",
       "client_secret": "YOUR_CLIENT_SECRET",
       "allow_from": []
+    },
+    "slack": {
+      "enabled": false,
+      "bot_token": "xoxb-YOUR-BOT-TOKEN",
+      "app_token": "xapp-YOUR-APP-TOKEN",
+      "allow_from": []
     }
   },
   "providers": {