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/home-manager.nix

@@ -1,5 +1,12 @@
 { pkgs, ... }:
 {
+  # Automatic garbage collection of old home-manager generations
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 7d";
+  };
+
   xdg.configFile."greenclip.toml".text = ''
     [greenclip]
       history_file = "~/.cache/greenclip.history"