feat(nightlight): scheduled blue-light filter via hyprsunset

Add nomarchy.nightlight (opt-in): a scheduled colour-temperature shift via
the HM services.hyprsunset module -- warm (.temperature, default 4000K) at
night, identity (no shift) by day, switching at .sunset / .sunrise. Two
time-based hyprsunset profiles, so hyprsunset handles the schedule and the
on-login state. New modules/home/nightlight.nix, imported in the home module.

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

modules/home/nightlight.nix

@@ -0,0 +1,25 @@
+# Night light — a scheduled blue-light filter via hyprsunset (Hyprland's own
+# gamma/temperature tool). Warm at night, identity (no shift) by day;
+# hyprsunset's time-based `profile` entries handle the schedule and pick the
+# right state on session start. Opt-in via nomarchy.nightlight.enable.
+#
+# The hyprsunset HM service module is provided by home-manager; this only
+# configures it. Override anything with plain services.hyprsunset.* options.
+{ config, lib, ... }:
+
+let
+  cfg = config.nomarchy.nightlight;
+in
+{
+  config = lib.mkIf cfg.enable {
+    services.hyprsunset = {
+      enable = true;
+      settings.profile = [
+        # Daytime: identity = no colour change.
+        { time = cfg.sunrise; identity = true; }
+        # Night: shift to the warm temperature.
+        { time = cfg.sunset; temperature = cfg.temperature; }
+      ];
+    };
+  };
+}