Ref T13600. When migrating observed repositories between cluster services, impact can be better controlled by fetching a copy of the repository on the target host before clusterizing it.

In particular, in the Phacility cluster, migrations are generally from one shared shard to one dedicated shard. It's helpful to perform these migrations synchronously without waiting for the cluster to sync in the background (helpful in the sense that there are fewer steps and fewer commands to run).

This supports an "--observe" mode to the internal "bin/services load-repository" workflow, which transfers repository data by refetching it from the remote rather than by getting it from the older host. This fetch occurs before cluster configuration is adjusted.

Ran locally as a sanity check, will apply in production.