github.event.workflow_run.head_branch is not populated for tag pushes
in Gitea Actions, causing the image tag to resolve to empty (-go suffix
with no version). Fix: build-go uploads the full image reference as a
one-line artifact; gitops-update downloads it via download-artifact@v4
with run-id from the workflow_run event.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>