ci: implement differential yamllint scan

Use vcs-diff-lint (with csdiff backend) to report only new warnings and
errors introduced in a PR.  This prevents older, pre-existing linter
violations from blocking new changes.

Same analyzer version is used for both the base and the head commit to
ensure consistent results.

See: infra/ansible#3143

.forgejo/workflows/yamllint-diff.yaml

@@ -0,0 +1,23 @@
+---
+name: Differential yamllint
+
+"on":
+  push:
+  pull_request:
+    branches: [main]
+
+jobs:
+  yamllint-job:
+    runs-on: infra-1
+    steps:
+      - name: Repository checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 100
+
+      - name: VCS Diff Lint
+        uses: https://github.com/fedora-copr/vcs-diff-lint-action@v1
+        id: VCS_Diff_Lint
+        with:
+          linter_tags: |
+            yamllint