ci: Add tests for better repo hygiene

[Unique-Divine]

Add Dotfiles CI Tests

This adds a lightweight GitHub Actions test workflow for the dotfiles repo and
adjusts local clipboard tests so the full just test command can run in CI
without requiring a WSL/Windows clipboard bridge.

Key Changes

• Add a Test GitHub Actions workflow that runs on pull requests and pushes to
  main, installs pinned Bun plus just, and executes just test.
• Gate clipboard integration tests on the presence of pbcopy and pbpaste, so
  machines without the local clipboard bridge skip those tests instead of
  failing unrelated CI.
• Keep the existing clipboard assertions unchanged on machines that do have the
  bridge, preserving local WSL coverage while making the repo testable on
  ubuntu-latest.

Appendix

Adding CI is useful here because the dotfiles repo now contains real TypeScript
automation, including skills syncing logic that can delete or copy directories.
Running just test on each PR gives fast feedback for that behavior, catches
regressions before merge, and makes future automation changes easier to review.