## Summary
Add Dependabot configuration to automatically keep GitHub Actions up to
date.
## Changes
Adds `.github/dependabot.yml` configured to:
- Check for GitHub Actions updates weekly
- Group all action updates together for easier review
- Use `ci` prefix for commit messages
## Why
As discussed in #958, this helps:
- Keep actions up to date with security patches automatically
- Handle Node runtime deprecations proactively (e.g., Node 20 → Node 24)
- Reduce manual maintenance burden
## Reference
Based on the pattern used in
[swift-nio](https://github.com/apple/swift-nio/blob/main/.github/dependabot.yml).
