Move Night Light onto a first-party nightlight service

Same treatment as Stay Awake: the indicator polled the toggle CLI over
a Process with a timer to paper over the race after clicking, and the
CLI ended by asking the shell to refresh every indicator over IPC. A new
omarchy.nightlight service owns hyprsunset instead - it probes the
temperature on startup, applies changes itself for in-shell toggles, and
answers on the nightlight IPC target. The indicator becomes a plain
binding. The CLI still drives hyprctl directly so keybindings, the menu,
and ssh work without the shell, but now just nudges the service to
re-probe since hyprsunset has no state file to watch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shell/plugins/README.md

@@ -32,6 +32,7 @@ User-installed plugins live alongside these conceptually but on disk under
 | Media         | `omarchy.media`           | `service`, `bar-widget` | `services/media/Service.qml`, `services/media/BarWidget.qml` |
 | Battery       | `omarchy.battery`         | `service`               | `services/battery/Service.qml`        |
 | Idle          | `omarchy.idle`            | `service`               | `services/idle/Service.qml`           |
+| Night light   | `omarchy.nightlight`      | `service`               | `services/nightlight/Service.qml`     |
 | Lock screen   | `omarchy.lock`            | `service`               | `lock/Service.qml`                    |
 | OSD           | `omarchy.osd`             | `panel`                 | `osd/Osd.qml`                         |
 | Polkit agent  | `omarchy.polkit`          | `service`               | `polkit/PolkitAgent.qml`              |