fix: merge codex-cli-nix into single valid flake

Two separate flake expressions were concatenated with a comma, causing
a syntax error. Merged them into one: added codex-cli-nix as an input
and devShells output alongside the existing nixosConfigurations.

https://claude.ai/code/session_01WbhyZRRuzioZn7F3NTuntf

flake.nix

@@ -11,13 +11,19 @@
       url = "github:schembriaiden/helium-browser-nix-flake";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    codex-cli-nix.url = "github:sadjow/codex-cli-nix";
   };
 
-  outputs = { nixpkgs, home-manager, helium-browser, ... }: {
-    nixosConfigurations.chexit = nixpkgs.lib.nixosSystem {
+  outputs = { nixpkgs, home-manager, helium-browser, codex-cli-nix, ... }:
+    let
       system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+    in
+    {
+      nixosConfigurations.chexit = nixpkgs.lib.nixosSystem {
+        inherit system;
         specialArgs = {
-        helium = helium-browser.packages.x86_64-linux.helium;
+          helium = helium-browser.packages.${system}.helium;
         };
         modules = [
           ./configuration.nix
@@ -30,20 +36,7 @@
           }
         ];
       };
-  };
-},
-{
-  inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    codex-cli-nix.url = "github:sadjow/codex-cli-nix";
-  };
 
-  outputs = { self, nixpkgs, codex-cli-nix }:
-    let
-      system = "x86_64-linux"; # or your system
-      pkgs = nixpkgs.legacyPackages.${system};
-    in
-    {
       devShells.${system}.default = pkgs.mkShell {
         buildInputs = [
           codex-cli-nix.packages.${system}.default