fix: use pre-installed yamllint from runner-tools image

yamllint is now pre-installed in the runner-tools Docker image.
Removes fragile pip install step that was failing silently.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/pr-checks.yaml

@@ -18,7 +18,6 @@ jobs:
       - name: YAML Lint
         run: |
           echo "=== YAML Lint ==="
-          pip install -q --break-system-packages yamllint
           yamllint -d "{extends: relaxed, rules: {line-length: {max: 200}}}" \
             environments/ argocd-apps/ apps/
           echo "YAML lint passed"