cron-source subscription bound
to an agent run. Creating a Loop records the binding and creates the backing
cron job that executes it — the box’s Hermes gateway cron on a Tower, or the
cloud sidecar cron.
Create a loop
schedule is a standard cron expression. The response includes the loop
subscription and the cron_job_id of the backing executor (null if no runtime
is reachable yet — the binding still persists and syncs later).