fix: add pnpm binary to nativeBuildInputs

pnpmConfigHook only sets up config, doesn't include the pnpm binary

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nix/package.nix

@@ -37,6 +37,7 @@ pkgs.stdenvNoCC.mkDerivation (finalAttrs: {
   nativeBuildInputs = [
     pkgs.makeBinaryWrapper
     pkgs.nodejs_24
+    pkgs.pnpm
     pkgs.pnpmConfigHook
   ];