chore: add Dependabot for GitHub Actions updates (#2266)

Configure Dependabot to keep GitHub Actions dependencies up to date
with weekly checks. Actions are already commit-SHA pinned across all
workflows; Dependabot will bump those SHAs when new versions are
released. Updates are grouped into a single PR to reduce noise.

Made-with: Cursor

Author: jonkoops

.github/dependabot.yml

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