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

@@ -11,39 +11,32 @@
       url = "github:schembriaiden/helium-browser-nix-flake";
       inputs.nixpkgs.follows = "nixpkgs";
     };
-  };
-
-  outputs = { nixpkgs, home-manager, helium-browser, ... }: {
-    nixosConfigurations.chexit = nixpkgs.lib.nixosSystem {
-      system = "x86_64-linux";
-      specialArgs = {
-        helium = helium-browser.packages.x86_64-linux.helium;
-      };
-      modules = [
-        ./configuration.nix
-        home-manager.nixosModules.home-manager
-        {
-          home-manager.useGlobalPkgs = true;
-          home-manager.useUserPackages = true;
-          home-manager.backupFileExtension = "hm-bak";
-          home-manager.users.chexit = import ./home.nix;
-        }
-      ];
-    };
-  };
-},
-{
-  inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     codex-cli-nix.url = "github:sadjow/codex-cli-nix";
   };
 
-  outputs = { self, nixpkgs, codex-cli-nix }:
+  outputs = { nixpkgs, home-manager, helium-browser, codex-cli-nix, ... }:
     let
-      system = "x86_64-linux"; # or your system
+      system = "x86_64-linux";
       pkgs = nixpkgs.legacyPackages.${system};
     in
     {
+      nixosConfigurations.chexit = nixpkgs.lib.nixosSystem {
+        inherit system;
+        specialArgs = {
+          helium = helium-browser.packages.${system}.helium;
+        };
+        modules = [
+          ./configuration.nix
+          home-manager.nixosModules.home-manager
+          {
+            home-manager.useGlobalPkgs = true;
+            home-manager.useUserPackages = true;
+            home-manager.backupFileExtension = "hm-bak";
+            home-manager.users.chexit = import ./home.nix;
+          }
+        ];
+      };
+
       devShells.${system}.default = pkgs.mkShell {
         buildInputs = [
           codex-cli-nix.packages.${system}.default