- Increase MAX_MESSAGE_LENGTH from 3,900 to 39,000 (Slack API allows 40k)
- Implement real typing indicator using assistant.threads.setStatus API
- Shows 'BotName is thinking...' next to the bot name in threads
- Auto-clears when the bot sends a reply
- Requires assistant:write or chat:write scope
- Falls back silently if scope unavailable (reactions still work)
- 4 new tests for typing indicator