mise: add usage dependency for shell completions

The generated script calls the separate `usage` CLI at completion time,
so add `pkgs.usage` to `home.packages` whenever mise is installed.
Stub it on Darwin test runs via `tests/darwinScrublist.nix`.

modules/programs/mise.nix

@@ -102,7 +102,11 @@ in
           The shell integration will not be added.
         '';
 
-    home.packages = lib.mkIf (cfg.package != null) [ cfg.package ];
+    home.packages = lib.mkIf (cfg.package != null) [
+      cfg.package
+      # The generated completions call the `usage` CLI at completion time.
+      pkgs.usage
+    ];
 
     xdg.configFile = {
       "mise/config.toml" = mkIf (cfg.globalConfig != { }) {

tests/darwinScrublist.nix

@@ -194,6 +194,7 @@ let
     "topgrade"
     "translate-shell"
     "tray-tui"
+    "usage"
     "vesktop"
     "vifm"
     "vim-vint"