[repo-assist] eng: add Dependabot configuration for NuGet and GitHub Actions

[github-actions]

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Summary

Adds .github/dependabot.yml to enable automated dependency tracking for both NuGet packages and GitHub Actions.

Why

The repository currently has no automated dependency update mechanism. Outdated packages can accumulate silently — Dependabot surfaces them as PRs so maintainers can review and merge at their own pace.

Changes

• NuGet: weekly check on Mondays, up to 5 open PRs, enhancement label
• GitHub Actions: weekly check on Mondays, up to 5 open PRs, enhancement label

Both ecosystems share the same schedule and PR limit to avoid queue build-up.

Test Status

✅ Build succeeded (0 warnings, 0 errors)
✅ 1,426 tests passed

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Task 4 – Engineering Investments

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Warning

Protected Files

This was originally intended as a pull request, but the patch modifies protected files. These files may affect project dependencies, CI/CD pipelines, or agent behaviour. Please review the changes carefully before creating the pull request.

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Protected files

• .github/dependabot.yml

To route changes like this to a review issue instead of blocking, configure protected-files: fallback-to-issue in your workflow configuration.

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