fix(ci): pass Go image tag from build to gitops via artifact

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>

.gitea/workflows/build.yaml

@@ -63,3 +63,11 @@ jobs:
             --build-arg BUILD_DATE=$(date -u +%Y-%m-%dT%H:%M:%SZ) \
             -t $IMAGE go/
           docker push $IMAGE
+          echo "$IMAGE" > /tmp/go-image-tag.txt
+
+      - name: Upload Go image tag for gitops workflow
+        uses: actions/upload-artifact@v4
+        with:
+          name: go-image-tag
+          path: /tmp/go-image-tag.txt
+          retention-days: 1