NixOS/nix — reverse-engineered prompt

Build me a package manager for Linux and other Unix systems, basically a Nix style tool that makes software installs reliable and reproducible. I want a command line app that can build and install packages in an isolated way, keep results deterministic, and let people describe environments declaratively instead of hand tweaking machines. It should feel solid for real development use, with clear documentation, tests, and a straightforward way to build it from source and try a simple first workflow.

Please make it look and behave like a serious open source tool, not a toy demo. Use the repo structure and docs to fill in the details, and check the current Nix manual and nix.dev if you need to. When you are done, I should be able to build it, run it from the terminal, and understand the basic first steps from the included docs.