ci(dependabot): group dependency update pull requests

[WH-2099]

Important

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2. Search existing issues and pull requests to confirm this change is not a duplicate
3. Open or identify the issue this pull request resolves or advances
4. Use a Conventional Commits title for this pull request, and mark breaking changes with !
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Related Issue

Closes #108

Summary

Configure Dependabot to group routine updates by package ecosystem instead of opening one pull request per dependency.

• group all uv dependency updates into one uv pull request
• group all GitHub Actions updates into one github-actions pull request
• keep the two ecosystems separate while preserving Conventional Commits PR titles for Dependabot updates

Validation: uv run prek run check-yaml --files .github/dependabot.yml; commit hooks also ran make tc successfully.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation

CLA Assistant has not prompted yet.