feat(gaming): add nomarchy.gaming.enable home-side window rule

Mirror of nomarchy.system.gaming.enable. When on, injects a Hyprland
windowrulev2 = fullscreen, class:^(steam_app_).*$ so games launched
through Steam grab the whole screen instead of opening windowed.

Gated via lib.mkIf so the rule is absent when the option is off
(AGENT.md guardrail: features must be option-gated). The rule is
appended to wayland.windowManager.hyprland.extraConfig (types.lines)
so it composes cleanly with the existing source-line entry point in
features/desktop/hyprland/default.nix.

Closes the "Gaming - Hyprland window rule" Next-column roadmap row.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/home/gaming.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.nomarchy.gaming;
+in
+{
+  config = lib.mkIf cfg.enable {
+    wayland.windowManager.hyprland.extraConfig = ''
+      windowrulev2 = fullscreen, class:^(steam_app_).*$
+    '';
+  };
+}