Added DM support: BigBrother can now talk back

Users can now DM the bot and get a response in character as BigBrother, powered by Llama 3.3 on Groq. The bot keeps a per-user conversation history (capped at 30 messages) so it actually remembers what you said earlier in the conversation. History lives in memory, so it resets on restart, intentional, didn’t feel worth the complexity of persisting it.
The interesting part was Slack’s permission system. Adding the im:history, im:read, and im:write scopes wasn’t enough on its own, you also need to separately subscribe to the message.im event under Event Subscriptions. The scopes let the bot act on DMs, the event subscription is what actually delivers them. Spent a while staring at logs with no “DM received” entries before finding that.