feat(nix): add initial Nix flake configuration and lock file for package management

flake.nix

@@ -0,0 +1,36 @@
+{
+  description = "Global packages for dotfiles";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [
+        "x86_64-darwin"
+        "aarch64-darwin"
+        "x86_64-linux"
+        "aarch64-linux"
+      ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f system);
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.buildEnv {
+            name = "dotfiles-packages";
+            paths = [
+              pkgs.nil
+              pkgs.nixfmt
+            ];
+          };
+        }
+      );
+    };
+}