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

packages/nunc/default.nix

@@ -0,0 +1,48 @@
+{ lib
+, swiftPackages
+, fetchgit
+,
+}:
+
+swiftPackages.stdenv.mkDerivation rec {
+  pname = "nunc";
+  version = "unstable-2026-04-15";
+
+  src = fetchgit {
+    url = "https://gitea.va.reichard.io/evan/Nunc.git";
+    rev = "95d95840aa0bc7b595293c6c5c9f53d120a90cca";
+    hash = "sha256-UXBKcEuNnVO7WzuvmVqXMiJH7uvjLkcWiqBO0BhynsU=";
+  };
+
+  nativeBuildInputs = [
+    swiftPackages.swift
+    swiftPackages.swiftpm
+  ];
+
+  buildInputs = [
+    swiftPackages.Foundation
+    swiftPackages.XCTest
+  ];
+
+  buildPhase = ''
+    runHook preBuild
+    swift build -c release
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p $out/bin
+    cp .build/release/Nunc $out/bin/nunc
+    runHook postInstall
+  '';
+
+  meta = {
+    description = "Minimal floating clock overlay for macOS";
+    homepage = "https://gitea.va.reichard.io/evan/Nunc";
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [ evanreichard ];
+    mainProgram = "nunc";
+    platforms = lib.platforms.darwin;
+  };
+}