cmd: config format migration strategy

[bobheadxi]

Right now if config is mismatched, Inertia just sort of quietly overwrites and throws away anything from a pervious format. We need to work out:

1. a strategy for executing format migrations
2. a command to do so

The new command chain from #577 makes things a little awkward for where to put a potential migrate command - I'd like to avoid introducing a inertia config, but inertia migrate seems too trivial for a top-level command, and inertia project migrate + inertia remote migrate seems awks (especially the latter)

Maybe migrations can be done silently by inertia as well