fix(ci): resolve image tag via Gitea API instead of artifact

upload/download-artifact@v4 is not supported on Gitea (GHES). Replace
with a direct Gitea API call in gitops-update: look up the tag name
whose commit SHA matches workflow_run.head_sha. Reverts the artifact
upload from build.yaml; no changes to build.yaml logic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -63,11 +63,3 @@ 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