CI base: GitHub Actions matrix (pwsh 7 + Pester)

[blindzero]

Goal

• Ensure every PR to main is validated automatically on supported OS targets.

Scope

• Add GitHub Actions workflow that runs:
  • pwsh 7
  • Invoke-Pester
• OS matrix:
  • windows-latest
  • ubuntu-latest
  • macos-latest
• Optional: publish test results / coverage artifact (only if stable)

Acceptance Criteria

• CI runs on every PR to main.
• Failing tests block merges.