Add dry-run arg for sync commands

[timabell]

It's hard to fix repo remotes once they are changed, adding a dry run might be useful.

Also clearer cli output on what changed would be good, currently has a lot of "processing" noise

[timabell]

we have a workaround for --read-remotes - backup / source-control the toml file

but for --write-remotes you'd have to zip up your entire repo folder, which sucks and you have no easy way of seeing what changed, so it's a bit of a trust-fall seeing what it'll do.

it works well but it could be nicer if anyone cares