instconf: take the origin and required-key helpers from ecore
Two copies of one function disagreed in this repo: hostFromOrigin returned an
error for a malformed origin and web's hostOf answered "localhost", which is a
guess that looks like an answer. Both are gone; the caller now names which half
it means, and both wanted OriginAuthority — a port is part of a sealed-URL host,
a JWT audience and the synthesized commit-author domain alike.
The startup checks become one Require, so an operator filling in a fresh
config.ini reads every missing key off one boot instead of one per restart. The
hook's internal-origin read becomes InternalOrigin, which falls back to the
external origin: an instance with only a public address is not misconfigured and
used to be refused. And the git-description mirror is wired only when git.sr.ht
has an API origin — web.Config already documented a nil Git as no mirroring, but
nothing produced one, so an instance without git.sr.ht met config.GetAPI's panic
on the first push.