Use home-manager.sharedModules for shared user config

Replace manual forEachUser pattern with built-in sharedModules for
applying config to all home-manager users. Add automatic garbage
collection of old generations (weekly, older than 7 days) and remove
the now-unnecessary expire-home-manager-generations justfile recipe.

Also update codex PR patch hash (upstream patch was modified).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/nix.nix

@@ -19,6 +19,7 @@
     home-manager.useGlobalPkgs = true;
     home-manager.useUserPackages = true;
     home-manager.backupFileExtension = "backup";
+    home-manager.sharedModules = [ ./home-manager.nix ];
 
     nix = rec {
       extraOptions = ''