Right now if you just run the code as normal for go programs:
go run coffer.go
it will just decrypt a 1Password.opvault/default folder at the given path. For now, you'll have to edit the path yourself since this code is still in a fledgling state. It will only decrypt band 0 for now, but it's trivial to decrypt the rest of the bands, and it will print out the contents of that band, namely, all the items that are stored in that band.
There is no guarantee that this doesn't trivially leak your password somehow, so please be careful with it.