.arcconfig

ui: manage logger instances and event filtering by core ui

The setup code in blackbox needs more tweaks since it has lots of black
magics. I'll fix them by follow-up patches.

To be clear, the goal of this series is to provide a proper way for command
server to install its own logger. I need it to debug in-memory repository
cache.

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