mirroring? #3

Following at://did:plc:3tiscg6kj4md5fkqq6mzzw6d/org.tangled.track.ticket/2 it'd be nice to auto-fetch changes to a canonical branch from a public remote to this private remote. We probably won't want to implement that ourselves, unless it would be rather trivial, but we could find, recommend, and maybe provide example config for something that already exists