Implement /admin command as specified in issue #50:
- Lists all admin user IDs for the current room
- Output format: 'Room Admins:\n- @user1\n- @user2'
- Shows 'No admins configured for this room.' if none exist
- Available to everyone (no permission check needed)
Changes:
- Add _find_username_by_user_id helper function
- Update cmd_admin to support listing when called with no args
- Update /help to include /admin command
Closes#50
