create a WithCwd abstraction in one place forever, because I'm tired of thinking about it when writing tests.  there are at least two other ways i'd rather do this, but this one will do.
belay that exception; it was helpfully generated by the fill-in-the-blanks hotkey but we've no need of it and will usually not be hiding on the other side of the Closable interface.
fix step missing in the middle of MdmAddCommand after the refactor.
get a basic test down hitting MdmAddCommand, with the help of WithCwd.

the way I'm using blocks here is kind of a travesty and a sham, but it's a convenient way to make it more obvious where cwd contexts begin and end.