Add Dependabot for GitHub Actions (#221)

## Summary
- Adds `.github/dependabot.yml` to keep GitHub Actions dependencies up to date automatically
- Configured for weekly checks

## Test plan
- [ ] Verify Dependabot starts creating PRs for action updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: nic-mxr

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"