CI: no image build on PRs; images are built only on release

The build-only image job on every PR goes: images are built when a
release is published (release.yml), so PRs don't wait on it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

.gitea/workflows/ci.yml

@@ -55,20 +55,6 @@ jobs:
           NISSE_OPERATOR_TOKEN: ${{ secrets.NISSE_OPERATOR_TOKEN }}
         run: uv run pytest -m "integration and not e2e" -rs
 
-  image:
-    # The agent's image builds (Dockerfile), so a broken one shows before a
-    # release does. On PRs only; nothing is pushed.
-    if: github.event_name != 'push'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Build the agent and test images
-        run: |
-          docker build --target agent -t nisse-dev:ci .
-          docker build --target test -t nisse-dev:ci-test .
-          docker run --rm nisse-dev:ci 2>&1 | grep -q "cannot start: no config file"
-
   e2e:
     # The end-to-end suite (scripts/e2e.sh): the whole agent against real
     # services, every role on a local model, so it spends nothing but keeps