Add Dependabot configuration

[Aaronontheweb]

Summary

• Adds weekly Dependabot checks for npm dependencies and GitHub Actions
• Groups minor/patch updates into single PRs to reduce noise
• Monitors GitHub Actions versions for security patches

What this enables

• Automated security vulnerability alerts (already active for public repos, this adds version update PRs)
• Keeps Astro/Starlight and build tooling current without manual intervention
• GitHub Actions pinned versions stay up to date