feat(discord): add group policy to control group respond behaviour

README.md

@@ -293,12 +293,18 @@ If you prefer to configure manually, add the following to `~/.nanobot/config.jso
     "discord": {
       "enabled": true,
       "token": "YOUR_BOT_TOKEN",
-      "allowFrom": ["YOUR_USER_ID"]
+      "allowFrom": ["YOUR_USER_ID"],
+      "groupPolicy": "mention"
     }
   }
 }
 ```
 
+> `groupPolicy` controls how the bot responds in group channels:
+> - `"mention"` (default) — Only respond when @mentioned
+> - `"open"` — Respond to all messages
+> DMs always respond when the sender is in `allowFrom`.
+
 **5. Invite the bot**
 - OAuth2 → URL Generator
 - Scopes: `bot`