feat: add flake.nix for reproducible dev environment

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -10,3 +10,5 @@ elm-stuff/
 *.zip
 packged/
 out-tsc/
+.direnv/
+.corepack/

flake.nix

@@ -0,0 +1,50 @@
+{
+  description = "wolfcola-devtools development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    systems.url = "github:nix-systems/default";
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      systems,
+      ...
+    }:
+    let
+      eachSystem = nixpkgs.lib.genAttrs (import systems);
+    in
+    {
+      devShells = eachSystem (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              nodejs_24
+              corepack
+              lefthook
+            ];
+
+            shellHook = ''
+              corepack enable --install-directory "$PWD/.corepack" >/dev/null 2>&1
+              export PATH="$PWD/.corepack:$PATH"
+
+              if [ ! -d node_modules ]; then
+                echo "node_modules missing — running pnpm install..."
+                pnpm install
+              elif [ pnpm-lock.yaml -nt node_modules ]; then
+                echo "pnpm-lock.yaml is newer than node_modules — running pnpm install..."
+                pnpm install
+              fi
+            '';
+          };
+        }
+      );
+
+      formatter = eachSystem (system: nixpkgs.legacyPackages.${system}.nixfmt);
+    };
+}