CI: GitHub Actions workflow for cross-platform releases

[pgivel]

Summary

Add a GitHub Actions workflow that builds and publishes release artifacts for Linux, macOS, and Windows when a version tag (v*) is pushed.

Approach

• Matrix build across ubuntu-latest, macos-latest, windows-latest
• Install Python + dependencies, run pyinstaller screen_ruler.spec on each
• Package binary + platform install script into:
  • .tar.gz for Linux and macOS
  • .zip for Windows
• Create a GitHub Release and upload all three archives

Prerequisites

• Repo must be public (for free Actions minutes), or the org must have Actions minutes available.

Related

Follows from #4 — the install scripts are already in place, this adds automated builds so users can download pre-built binaries from the Releases page.