nix: setup nix flake

flake.nix

@@ -0,0 +1,42 @@
+{
+  description = "PayKit - Open payments orchestration for modern SaaS";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+  }:
+    flake-utils.lib.eachDefaultSystem (
+      system: let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            # Node.js 22 as specified in .nvmrc
+            nodejs_22
+
+            # pnpm as specified in package.json (nixpkgs has 10.4.1)
+            pnpm
+
+            # Build tools
+            turbo
+
+            # Formatters and linters
+            oxfmt
+            oxlint
+
+            # Additional dev tools
+            git
+
+            # Shell utilities
+            jq
+          ];
+        };
+      }
+    );
+}