General-purpose Forgejo pull-mirror config (services.hyperhive.forge.mirrors:
list of {upstream, dest}). When CI is enabled, auto-append an actions/checkout
mirror + point forgejo DEFAULT_ACTIONS_URL at this instance so CI's
actions/checkout@vN resolves on loopback (immune to host-resolver blips, #2072).
Seed oneshot (creates the dest orgs + pull-mirrors via the migrate API) is the
next commit.