docs: move explanations into short module headers

Non-obvious context sits in a brief header at the top of a module, everything else uses single-line comments. niri binds use spawn-sh strings, so neon only overrides the command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hosts/argon/niri.nix

@@ -1,4 +1,5 @@
 # argon: desktop with two 1440p monitors side by side
+
 {
   wayland.windowManager.niri.settings = {
     input = {

hosts/default/configuration.nix

@@ -1,4 +1,4 @@
-# 0x29a nixos conifg
+# 0x29a nixos config
 
 { pkgs, ... }:
 

hosts/neon/niri.nix

@@ -1,17 +1,7 @@
-# neon: laptop, built-in display is auto-configured by niri
+# neon: laptop, the built-in display is auto-configured by niri
+
 { lib, ... }:
 
-let
-  brightness = step: {
-    _props.allow-when-locked = true;
-    spawn = [
-      "brightnessctl"
-      "--class=backlight"
-      "set"
-      step
-    ];
-  };
-in
 {
   wayland.windowManager.niri.settings = {
     input.touchpad = {
@@ -26,9 +16,9 @@ in
       "Ctrl+Print".screenshot-screen = { };
       "Mod+Shift+P".power-off-monitors = { };
 
-      # finer brightness steps; mkForce, since spawn lists would otherwise be concatenated
-      "XF86MonBrightnessUp" = lib.mkForce (brightness "+1%");
-      "XF86MonBrightnessDown" = lib.mkForce (brightness "1%-");
+      # finer brightness steps
+      "XF86MonBrightnessUp".spawn-sh = lib.mkForce "brightnessctl --class=backlight set +1%";
+      "XF86MonBrightnessDown".spawn-sh = lib.mkForce "brightnessctl --class=backlight set 1%-";
     };
   };
 }