.arcconfig

registrar: host "dynamicdefault" constant by configitem object

This is the common pattern seen in the other registrar classes.

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