Who a migration job runs as: the admin who started its run. The AshOban.ActorPersister of
MishkaCmsMigration.MigrationRun's triggers keeps the user's id in the job (store/1) and
loads the user with their permissions when the job runs (lookup/1), so a target writes as that
admin under the CMS's own policies, with the permissions they hold then. A user who is gone
fails the attempt.
lookup/1 reads the user as the system (authorize?: false): there is no actor yet to read as,
since finding one is what it is for. It is one of the places the extension acts as the system,
with MishkaCmsMigration.Workers.Helpers.Discarded, which runs outside any job, and
MishkaCmsMigration.Resources.Notifiers.Steps, which writes a run's log.