gaming: move home module to gaming/ dir, add moonlight client toggle

Mirror the hosts/common/features/gaming/ layout on the home side: turn the
single gaming.nix into a gaming/ directory. Moonlight (the client
counterpart to the Sunshine host) becomes moonlight.nix, gated by its own
custom.gaming.moonlight option so a host can install just the streaming
client without the full launcher/Proton toolkit (custom.gaming.enable).

- gaming.nix -> gaming/default.nix (imports moonlight.nix)
- gaming/moonlight.nix: custom.gaming.moonlight -> moonlight-qt
- arrakis: import gaming module, set custom.gaming.moonlight = true
  (replaces the one-off home.packages line)
- konishi: update import path to the directory

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

home/arrakis/default.nix

@@ -14,11 +14,15 @@
     ../common/features/downloads-cleanup.nix
     ../common/programs/3d.nix
     ../common/programs/godot.nix
+    ../common/programs/gaming
     ./monitors.nix
   ];
 
   custom.hyprland.cursorSize = 32;
 
+  # Moonlight: client for the Sunshine server on konishi (stream games to this laptop).
+  custom.gaming.moonlight = true;
+
   wayland.windowManager.hyprland.settings = {
     exec-once = [
     ];