Add infra on-call members to each new incident channel

New incident channels can open without on-call members, leaving responders short on context. The channel will include the active on-call squad so triage can begin within minutes.

Overview

Incidents demand immediate, coordinated attention — missing the right people at channel creation delays triage and extends outage windows. This flow adds the active on-call crew into each incident channel within minutes so responders have access and context to act quickly.