ci: add dependabot github-actions updates (#9)

Create .github/dependabot.yml with 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

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      all:
+        patterns:
+          - "*"