docs: add SECURITY.md

[zichen0116]

Summary

This PR adds a SECURITY.md file to the project.

Changes

• Added SECURITY.md with comprehensive security policy including:
  • Supported versions table
  • Vulnerability reporting guidelines
  • Response timeline expectations
  • Security best practices for contributors

Motivation

Having a proper security policy helps:

• Provide clear instructions for reporting vulnerabilities
• Set expectations for response timelines
• Encourage responsible disclosure
• Improve overall project security posture

Checklist

• File follows standard security policy conventions
• Content is appropriate for the project
• No breaking changes