fix(slack): persist daily-brief anchor into state
Follow-up on MR!15: propagate anchor value into state on skip so save_state writes today's date even when load_state returned empty (Colima cache miss). Prevents duplicate brief on same-day restart.
Follow-up on MR!15: propagate anchor value into state on skip so save_state writes today's date even when load_state returned empty (Colima cache miss). Prevents duplicate brief on same-day restart.