ADR-0022: Move Announcement Entity Architecture to Accepted

ADR-0022 (Announcement Entity Architecture) is written and staged at brain/projects/rsvp-system/architecture/ADR-0022-announcement-entity-architecture.md. Action: review ADR-0022, confirm Option A (composed entities with references) is the approach, and update status to Accepted. Note: source doc at gluebox.com/node/89 labeled this ADR-0018, but that number is taken by Recurring Events Admin Views. Filed as ADR-0022.