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-rs/default.nix

@@ -0,0 +1,42 @@
+{ pkgs, monorep-deps ? [], ... }:
+let
+  env = {
+    PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig:$PKG_CONFIG_PATH";
+  };
+in
+rec {
+  package = pkgs.rustPlatform.buildRustPackage {
+    inherit env;
+    pname = "codex-rs";
+    version = "0.1.0";
+    cargoLock.lockFile = ./Cargo.lock;
+    doCheck = false;
+    src = ./.;
+    nativeBuildInputs = with pkgs; [
+      pkg-config
+      openssl
+    ];
+    meta = with pkgs.lib; {
+      description = "OpenAI Codex command‑line interface rust implementation";
+      license = licenses.asl20;
+      homepage = "https://github.com/openai/codex";
+    };
+  };
+  devShell = pkgs.mkShell {
+    inherit env;
+    name = "codex-rs-dev";
+    packages = monorep-deps ++ [
+      pkgs.cargo
+      package
+    ];
+    shellHook = ''
+      echo "Entering development shell for codex-rs"
+      alias codex="cd ${package.src}/tui; cargo run; cd -"
+      ${pkgs.rustPlatform.cargoSetupHook}
+    '';
+  };
+  app = {
+    type = "app";
+    program = "${package}/bin/codex";
+  };
+}