sandbox: on checkout, update the local project file #1076

When checking out from a to b, we should write the state of a into a.yaml. That way no changes will be lost.

This is related to the save command - the idea that I want to manually update my local project, but not push it to the app.

Note that updating the local project file always introduces a risk of conflict from lightning at the next fetch. We just have to cross that bridge when we get to it