Add Homebrew formula and release workflow

- Formula/scriptr.rb: Homebrew formula for macOS (arm64/x86_64) and Linux (arm64/x86_64), placeholder SHAs until first release
- .github/workflows/release.yml: CI workflow builds 4 targets on tag push, creates GitHub release, regenerates formula with real SHA256s
- justfile: added `release version` recipe to bump Cargo.toml, commit, tag, and push
- README.md: added Homebrew as the first install option