.arcconfig

context: reintroduce `ctx.descendant` as deprecated

Reintroduce `ctx.descendant` to help extensions authors update their
extensions in order to use the new API.

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