ci: commit the apk build, packaging and mirror-trigger files

These three were written but never committed, so the whole publishing chain
was dead from its first link: with no .build.yml on sourcecraft there is none
on the git.srht.bigb.es mirror either, so the push hook has no manifest to
submit, no build ever ran, and compare.sr.ht has never appeared in the apk
index at repo.bigb.es — which today holds spec.sr.ht alone.

The code itself was mirroring fine, which is what made this hard to see: the
sourcehut side sits on the current commit, so everything looks configured
until you ask git which of these files it actually knows about.

.sourcecraft/webhooks.yaml is in the same state and has the same consequence
one level up: the gitsync webhook was never registered, so the mirror has been
riding its hourly safety-net poll rather than updating on push.

The build secret apk-ci-s3 the publish task needs now exists on the account,
so nothing else blocks the first build.