Add contributor and security documentation

[ashishsinghbora]

Problem Statement

New contributors currently need to understand the project architecture and safety requirements before making changes.

Security reporting also needs a clear process.

Proposed Solution / Enhancement

Add:

• CONTRIBUTING.md
• SECURITY.md

CONTRIBUTING.md should explain:

• Project architecture
• Development setup
• Running tests
• Writing tests
• Pull request expectations
• Coding conventions

SECURITY.md should explain:

• How to report security issues
• What information should be included
• What should not be publicly disclosed

Architectural Layer Impacted

Documentation / Open Source / Security

Alternatives Considered

Leaving contributor and security guidance undocumented increases onboarding time and can result in unsafe public disclosure of vulnerabilities.