feat(nunc): add nunc floating clock overlay package and service

- Add packages/nunc derivation built from Gitea with swiftPackages
- Add home module reichard.services.nunc with configurable settings
  and launchd agent for macOS
- Enable nunc service on mac-va-mbp-work
- Tweak mac-va-mbp-work defaults: hide menu bar, disable auto
  brightness, hide Finder desktop icons

systems/aarch64-darwin/mac-va-mbp-work/default.nix

@@ -28,12 +28,20 @@
       KeyRepeat = 2;
       NSWindowShouldDragOnGesture = true;
       AppleICUForce24HourTime = true;
+      _HIHideMenuBar = true;
     };
     WindowManager = {
       HideDesktop = true;
     };
+    finder = {
+      CreateDesktop = false;
+    };
   };
 
+  system.activationScripts.postActivation.text = ''
+    sudo defaults write /Library/Preferences/com.apple.iokit.AmbientLightSensor "Automatic Display Enabled" -bool false
+  '';
+
   security.pam.services.sudo_local.touchIdAuth = true;
 
   reichard = { };