feat(create): scaffold repositories via templates with preview and mirror setup

https://chatgpt.com/share/693f5bdb-1780-800f-a772-0ecf399627fc

templates/default/flake.nix.j2

@@ -0,0 +1,11 @@
+{
+  description = "{{ repository }}";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  outputs = { self, nixpkgs }:
+  let system = "x86_64-linux"; pkgs = import nixpkgs { inherit system; };
+  in {
+    devShells.${system}.default = pkgs.mkShell {
+      packages = with pkgs; [ python312 python312Packages.pytest python312Packages.ruff ];
+    };
+  };
+}