License under GPL-3.0-or-later

Add LICENSE, ship it in release archives, and declare the licence in
the nfpm packages and Homebrew formula.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/homebrew/mapsize.rb.in

@@ -5,6 +5,7 @@
 class Mapsize < Formula
   desc "Interactive terminal disk usage analyser with a navigable treemap"
   homepage "https://dixon.cx"
+  license "GPL-3.0-or-later"
   url "https://github.com/andydixon/mapsize/archive/refs/tags/v@VERSION@.tar.gz"
   sha256 "@SHA256@"
   head "https://github.com/andydixon/mapsize.git", branch: "master"