Add lint workflow

.github/workflows/lint.yml

@@ -0,0 +1,26 @@
+name: PlantUML images
+
+on:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  drawio:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Verify number of diagrams
+        run: |
+          rc=0
+
+          while read -r file; do
+            if [[ $(grep "<diag" "$file" | wc -l) -ne 1 ]]; then
+              >&2 echo "$file should contain exactly one diagram"
+              rc=1
+            fi
+          done < <(find . -name "*.drawio")
+
+          exit $rc