Relicense project to AGPL-3.0-only
- Add official AGPL-3.0 LICENSE from gnu.org
- Add CONTRIBUTING.md with DCO sign-off requirement
- Update pyproject.toml license field and classifier
- Update README with sol pbc attribution and AGPL license section
- Update docs/VENDOR.md to reference AGPL-compatible licenses
- Add SPDX-License-Identifier headers to all 188 source files
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>