chore: add Dependabot config and a PR build workflow

[prmeyn]

Summary

• Adds .github/dependabot.yml covering: nuget,npm,github-actions, all on a weekly schedule.
• NuGet project folders are listed individually rather than the repo root, so the solution doesn't make Dependabot discover every dependency twice. SMSwitch hit that problem.
• Semver-major updates are ignored for every ecosystem, so a major bump stays a deliberate change. Security updates still come through.
• NuGet updates are grouped as sibling prmeyn packages → Microsoft/System → everything else.
• Adds .github/workflows/ci.yml (job build), so Dependabot's PRs are built before merging. The build runs with `/warnaserror` because it is currently at 0 warnings.

Dependabot alerts and automated security updates are being enabled on this repo through the API alongside this PR.

Test plan

• The Dependabot config check on this PR passes (GitHub validates dependabot.yml)
• The build check is green
• After merging, Insights → Dependency graph → Dependabot lists each configured directory

🤖 Generated with Claude Code