docs(template): ship opt-in features as commented examples downstream

Convention: an off-by-default feature that needs nomarchy.* settings in the
downstream files should appear commented-out in templates/downstream/{home,
system}.nix, so a new install enables it by uncommenting + tweaking rather
than hunting the docs for the option name. Apply it now: night light,
nomarchy.monitors, per-device keyboard layouts (home.nix) and power
management (system.nix). Also fix the stale keyboard.layout note
(console/LUKS follow xkb automatically now). Documented in README S6.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/downstream/system.nix

@@ -18,9 +18,18 @@
     extraGroups = [ "wheel" "networkmanager" "video" "input" ];
   };
 
-  # Examples:
+  # ── Overrides (uncomment to change) ─────────────────────────────────
   # nomarchy.system.greeter.enable = false;   # bring your own login manager
   # environment.systemPackages = [ pkgs.htop ];
 
+  # ── Opt-in features — uncomment and tweak to enable ─────────────────
+  # nomarchy.system.power = {                  # active power management (laptops)
+  #   enable = true;
+  #   backend = "ppd";                         #   "ppd" (default) or "tlp"
+  #   laptop = true;                           #   required by batteryChargeLimit
+  #   batteryChargeLimit = 80;                 #   cap charging for longevity
+  #   thermal.enable = true;                   #   thermald (Intel CPUs)
+  # };
+
   system.stateVersion = "26.05";
 }