Add verbatim GPL-3.0 text (copied from /usr/share/common-licenses/GPL-3). Repo is now licensed GPL-3.0-or-later. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01P8QfqokDoggsPVhEp7FnCa