Send closed beta acceptance alerts to onboarding channel

Closed beta acceptances arrive without mapped owners, so coordinators don't know who to contact and onboarding stalls. Coordinators get owner mentions and alerts to start outreach the same day.

Overview

When closed beta acceptances land without owner mapping, manual routing delays outreach and provisioning. This flow finds the right owner, composes owner mentions, and posts a clear channel alert so coordinators can start outreach the same day.