Enable Dependabot for npm and GitHub Actions

[mcgroarty]

Adds .github/dependabot.yml to enable automatic dependency updates.

What this enables

• npm ecosystem — weekly updates for package.json dependencies (handles the pnpm lockfile natively).
  • Astro / @astrojs/* packages grouped into a single PR to reduce noise.
  • All dev dependencies grouped into a single PR.
• github-actions ecosystem — weekly updates for action versions referenced in .github/workflows/ci.yml and deploy.yml.

Settings

• PR limit: 10 (npm)
• Assignee: @mcgroarty on both ecosystems

Once merged, Dependabot picks up the config automatically — no repo setting changes required.

[Copilot]

Pull request overview

Adds a Dependabot configuration file to enable automated weekly dependency update PRs for both JavaScript dependencies and GitHub Actions used by this repository.

Changes:

• Add weekly Dependabot updates for the npm ecosystem at the repository root (compatible with the existing pnpm-lock.yaml).
• Configure grouping to reduce PR noise (Astro-related updates grouped; all dev-dependency updates grouped).
• Add weekly Dependabot updates for the github-actions ecosystem (covers workflow actions under .github/workflows/).

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