feat: PyInstaller builds for Windows, Linux, and macOS

[HN-Tran]

Summary

• Adds a PyInstaller hook (hooks/hook-magic.py) to bundle python-magic-standalone's shared libraries and data files into the executable
• Adds a GitHub Actions workflow (.github/workflows/build.yml) that builds Linux, Windows, and macOS executables on tag push or manual dispatch
• On tag push, automatically creates a GitHub Release with all three platform binaries attached (with auto-generated release notes)

Test plan

• Triggered workflow manually on fork — all 3 platform builds succeeded: https://github.com/HN-Tran/python-wayback-machine-downloader/actions/runs/23326178541
• Linux executable tested locally: -h and actual download both work
• Test Windows and macOS executables
• Push a test tag to verify GitHub Release creation

[bitdruid]

A few changes needed to match the requirements: only trigger when a release is published, support x.y.z tags (no v prefix), and don't overwrite manually written release notes.