feat: initial Moongreet greeter implementation

greetd-Greeter für Wayland mit Python + GTK4 + gtk4-layer-shell.
Enthält IPC-Protokoll, User/Session-Erkennung, Power-Actions,
komplettes UI-Layout und 36 Tests (Unit + Integration).

config/moongreet.toml

@@ -0,0 +1,9 @@
+# ABOUTME: Optional configuration for the Moongreet greeter.
+# ABOUTME: Background image and other visual settings.
+
+[appearance]
+# background = "/usr/share/backgrounds/moonarch.jpg"
+
+[behavior]
+# show_user_list = true
+# default_session = "Hyprland"