ci: let Dependabot keep the pinned workflow actions current (#52)

The actions are pinned to commit SHAs, and nothing proposes updates to
them: the Node 20 runtime several of them still declare is already being
forced to Node 24, with a warning on every run. Dependabot now checks them
monthly and groups all action updates into one pull request.

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  # Workflow actions are pinned to commit SHAs; this keeps the pins current
+  # (new releases, runtime moves such as Node 20 -> 24) in one PR a month.
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    groups:
+      actions:
+        patterns: ["*"]
+    commit-message:
+      prefix: ci