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/gitops-update.yaml

@@ -39,10 +39,10 @@ jobs:
       GITEA_TOKEN: ${{ secrets.GITOPS_TOKEN }}
 
     steps:
-      - name: Install git, curl, ca-certificates
+      - name: Install git, curl, ca-certificates, jq
         run: |
           apt-get update -qq
-          apt-get install -y --no-install-recommends git curl ca-certificates
+          apt-get install -y --no-install-recommends git curl ca-certificates jq
 
       - name: Install tea
         run: |
@@ -62,12 +62,17 @@ jobs:
         id: resolve
         run: |
           if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
-            GIT_TAG="${{ inputs.tag }}"
+            IMAGE="gitea.home.hrajfrisbee.cz/${{ github.repository }}:${{ inputs.tag }}-go"
           else
-            # workflow_run: use the ref name of the triggering workflow (the pushed git tag).
-            GIT_TAG="${{ github.event.workflow_run.head_branch }}"
+            # workflow_run: head_branch is not populated for tag pushes in Gitea Actions.
+            # Look up the tag name that points to the triggering commit SHA via the API.
+            SHA="${{ github.event.workflow_run.head_sha }}"
+            GIT_TAG=$(curl -fsSL \
+              -H "Authorization: token ${GITEA_TOKEN}" \
+              "https://gitea.home.hrajfrisbee.cz/api/v1/repos/${{ github.repository }}/tags?limit=50" \
+              | jq -r --arg sha "$SHA" '.[] | select(.commit.sha == $sha) | .name')
+            IMAGE="gitea.home.hrajfrisbee.cz/${{ github.repository }}:${GIT_TAG}-go"
           fi
-          IMAGE="gitea.home.hrajfrisbee.cz/${{ github.repository }}:${GIT_TAG}-go"
           echo "image=${IMAGE}" >> "$GITHUB_OUTPUT"
           echo "Resolved image: ${IMAGE}"