fix(slack): daily brief dedup with process anchor
Same Colima/virtiofs bug as trends: load_state returns empty last_daily_brief_date after rebuild → re-posts every 5 min tick. Apply module-level anchor backstop.
Same Colima/virtiofs bug as trends: load_state returns empty last_daily_brief_date after rebuild → re-posts every 5 min tick. Apply module-level anchor backstop.