restructure flake for codex-rs (#888)

Right now since the repo is having two different implementations of
codex, flake was updated to work with both typescript implementation and
rust implementation

codex-cli/default.nix

@@ -0,0 +1,43 @@
+{ pkgs, monorep-deps ? [], ... }:
+let
+  node = pkgs.nodejs_22;
+in
+rec {
+  package = pkgs.buildNpmPackage {
+    pname       = "codex-cli";
+    version     = "0.1.0";
+    src         = ./.;
+    npmDepsHash = "sha256-3tAalmh50I0fhhd7XreM+jvl0n4zcRhqygFNB1Olst8";
+    nodejs      = node;
+    npmInstallFlags = [ "--frozen-lockfile" ];
+    meta = with pkgs.lib; {
+      description = "OpenAI Codex command‑line interface";
+      license     = licenses.asl20;
+      homepage    = "https://github.com/openai/codex";
+    };
+  };
+  devShell = pkgs.mkShell {
+    name        = "codex-cli-dev";
+    buildInputs = monorep-deps ++ [
+      node
+      pkgs.pnpm
+    ];
+    shellHook = ''
+      echo "Entering development shell for codex-cli"
+      # cd codex-cli
+      if [ -f package-lock.json ]; then
+        pnpm ci || echo "npm ci failed"
+      else
+        pnpm install || echo "npm install failed"
+      fi
+      npm run build || echo "npm build failed"
+      export PATH=$PWD/node_modules/.bin:$PATH
+      alias codex="node $PWD/dist/cli.js"
+    '';
+  };
+  app = {
+    type    = "app";
+    program = "${package}/bin/codex";
+  };
+}
+