Add GitHub Actions workflow that triggers on version tags (v*) to
automatically build release artifacts and create GitHub releases.
- Build matrix for Linux and Windows (amd64)
- Create proper archives (.tar.gz and .zip)
- Inject version via ldflags at build time
- Extract release notes from CHANGELOG.md
- Detect pre-release tags (-alpha, -beta, -rc)
- Add Version variable to main.go for runtime version display
