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>

nfpm.yaml

@@ -4,6 +4,7 @@ version: ${VERSION}
 maintainer: Andy Dixon <andy@dixon.cx>
 description: Interactive full-terminal disk usage analyser with a live treemap.
 homepage: https://github.com/andydixon/mapsize
+license: GPL-3.0-or-later
 contents:
   - src: bin/release/pkg/mapsize
     dst: /usr/bin/mapsize