v0.7.0

feat: prebuilt binary releases (no Rust needed to install)

- .github/workflows/release.yml: on a v* tag, CI builds a binary on real
  macOS (arm64 + x64), Linux (x64), and Windows (x64) runners and attaches
  them to the GitHub Release.
- install.sh / install.ps1: download the prebuilt binary for the user's
  OS/arch from the latest release; fall back to 'cargo install' only when run
  inside a clone with Rust.
- README: prebuilt curl|bash / irm|iex one-liners first, source build second.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>