ClientUtils.Harness.Onboarding.FileIO (client_utils v0.1.30)

The default way onboarding touches a working copy: the real filesystem.

Onboarding is mostly decisions, but it has to write four or five things, and the code that decides should not have to know how. This is the plain answer — File and System.cmd/3, rooted at a directory — and it is what a generated application gets without configuring anything.

Its handle is the root, because a directory is all this needs. A host with its own filesystem abstraction passes {ThatModule, handle} instead; the callback names here are CodeMySpec.Environments' names precisely so that host writes no adapter — see ClientUtils.Harness.Onboarding.Port.