ci: add dependabot github-actions updates (#23)

Add a github-actions package ecosystem entry so third-party actions
pinned with mutable tags (e.g. @v4) get auto-updated weekly.

Co-authored-by: Kim Harjamäki <kim.harjamaki@prosimo.fi>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

.github/dependabot.yml

@@ -8,3 +8,11 @@ updates:
       all:
         patterns:
           - "*"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      all:
+        patterns:
+          - "*"