.arcconfig

ui: add debug commands to test interactive prompt

Interactive operations aren't easily covered by tests. So let's add commands
to test them manually.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}