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

homes/aarch64-darwin/evanreichard@mac-va-mbp-work/default.nix

@@ -40,6 +40,10 @@ in
       };
     };
 
+    services = {
+      nunc = enabled;
+    };
+
     security = {
       sops = enabled;
     };