fix: use --break-system-packages for pip install yamllint

PEP 668 blocks pip install in externally-managed Python environments
(Debian/Ubuntu). Adding --break-system-packages flag to fix CI.

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

.gitea/workflows/pr-checks.yaml

@@ -18,9 +18,9 @@ jobs:
       - name: YAML Lint
         run: |
           echo "=== YAML Lint ==="
-          pip install -q yamllint 2>/dev/null
+          pip install -q --break-system-packages yamllint
           yamllint -d "{extends: relaxed, rules: {line-length: {max: 200}}}" \
-            environments/ argocd-apps/ apps/ 2>&1
+            environments/ argocd-apps/ apps/
           echo "YAML lint passed"
 
       - name: Validate ArgoCD Manifests