Home Manager: never set up a laptop twice

A Mac user shows how to rebuild their entire development environment from a Git-backed Home Manager config on a fresh machine.

After installing Nix with the Determinate Systems installer, they clone the repo and run `home-manager switch` to restore packages, shell settings, aliases, and tools.

The setup is pinned with `flake.nix` and `flake.lock`, making the environment reproducible and easy to recover.

The workflow uses Home Manager alone on macOS, without `nix-darwin`, to manage the user-level configuration.