ci: add validation workflow

.forgejo/workflows/validate-shared-actions.yml

@@ -0,0 +1,23 @@
+name: validate-shared-actions
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+permissions:
+  contents: read
+
+jobs:
+  validate-shared-actions:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Validate shared action metadata
+        uses: mpalmer/action-validator@v0.9.0
+        with:
+          version: 0.9.0
+          patterns: |
+            :(glob)**/action.yml
+            :(glob)**/action.yaml