chore: add security, contribution, and community guidelines

Introduce project governance and community documentation to support secure,
responsible, and collaborative development.

This commit adds:
- SECURITY.md defining vulnerability reporting and responsible disclosure
- CODE_OF_CONDUCT.md establishing community behavior standards
- CONTRIBUTING.md outlining contribution workflow and expectations
- SUPPORT.md describing how users can get help and report issues

These additions improve project maturity, clarify contributor expectations,
and align the repository with open-source best practices without impacting
runtime behavior or public APIs.