feat: implement hybrid declarative state with automatic Nix sync

docs/MIGRATION.md

@@ -122,6 +122,12 @@ create minimal versions to start:
 ```nix
 { pkgs, ... }:
 {
+  imports = [
+    # Machine-managed state (theme, font, toggles).
+    # UI scripts update this file automatically.
+    ./nomarchy-state.nix
+  ];
+
   home.packages = with pkgs; [
     firefox
     xfce.thunar
@@ -144,9 +150,17 @@ create minimal versions to start:
 }
 ```
 
+Since you are migrating manually, you'll need to generate the initial
+`nomarchy-state.nix` so the import doesn't fail. Run this once:
+
+```bash
+# Seed the initial Nix state from the defaults
+nomarchy-sync-nix-state
+```
+
 Move any user/services/packages you had in `configuration.nix` over to
 `system.nix`. Do **not** redefine things Nomarchy already provides (display
-manager, Hyprland, PipeWire, NetworkManager) unless you want to override
+...manager, Hyprland, PipeWire, NetworkManager) unless you want to override
 them — see the [conflicts](#conflicts-to-resolve-before-rebuild) section.
 
 ## 3. (Optional) Pick up hardware-specific tuning