Update license from PolyForm Noncommercial to GPL-3.0

- Changed LICENSE to GNU General Public License v3.0
- Updated README.md badge and license section
- Provides strong copyleft protection while allowing open collaboration
- Requires derivative works to be open source under same terms

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Ziforge