openspec: migrate from pnpm_9 to pnpm_11

pnpm 9.x reached upstream end-of-life on 2026-04-30. Migrate to pnpm_11
and bump fetchPnpmDeps fetcherVersion 3 -> 4 since the package requires a
rebuild anyway.

Part of #529285.

pkgs/by-name/op/openspec/package.nix

@@ -3,7 +3,7 @@
   stdenvNoCC,
   fetchFromGitHub,
   nodejs,
-  pnpm_9,
+  pnpm_11,
   fetchPnpmDeps,
   pnpmConfigHook,
   makeWrapper,
@@ -24,15 +24,15 @@ stdenvNoCC.mkDerivation (finalAttrs: {
 
   pnpmDeps = fetchPnpmDeps {
     inherit (finalAttrs) pname version src;
-    pnpm = pnpm_9;
-    fetcherVersion = 3;
-    hash = "sha256-9s2kdvd7svK4hofnD66HkDc86WTQeayfF5y7L2dmjNg=";
+    pnpm = pnpm_11;
+    fetcherVersion = 4;
+    hash = "sha256-p44ctVCA3d1CXoq+zzhswVqhScF23ZhvpLUVVcrgQlM=";
   };
 
   nativeBuildInputs = [
     nodejs
     pnpmConfigHook
-    pnpm_9
+    pnpm_11
     makeWrapper
     installShellFiles
   ];