Source just from upstream GitHub flake instead of nixpkgs-unstable

[Copilot]

This updates the Nix configuration to fetch just from its upstream GitHub flake, aligning with the project docs and avoiding reliance on nixpkgs-unstable for this tool. The change keeps package resolution system-aware across Linux and Darwin configs.

• Problem addressed

  • just was previously installed via unstable.just, not from the upstream flake source recommended in the docs.

• Flake input wiring

  • Added just as a top-level flake input:
    • just.url = "github:casey/just";
  • Passed just through extraSpecialArgs so Home Manager modules can consume it in both target configurations.

• Package source change

  • Replaced unstable.just in modules/packages.nix with the upstream flake package:
    • just.packages.${pkgs.system}.default

# flake.nix
inputs.just.url = "github:casey/just";

# modules/packages.nix
let
  justPackage = just.packages.${pkgs.system}.default;
in {
  home.packages = with pkgs; [
    justPackage
  ];
}