Addresses the review warning: git rev-parse confirms the tag is
present in the local repo after git fetch --tags before git-cliff
or git tag --sort= runs against it. Fails fast with a clear error
if the tag is missing rather than silently generating an incomplete
changelog.
The changelog and build-* jobs all fan out from autotag in parallel.
Build jobs create the Gitea release with 'curl ... || true', but the
changelog job was trying to GET the release before any build job had
run, reliably failing with 'Could not find release for tag vX.Y.Z'.
Fix: changelog job owns release creation. It creates the release with
the git-cliff body if it does not exist, or patches the body if a
prior run already created it. Build jobs continue using their existing
create-or-skip + upload pattern unchanged.
