Add CODE_OF_CONDUCT.md

[JosephDoUrden]

Summary

Adopt the Contributor Covenant Code of Conduct to establish community standards.

What needs to be done

• Create CODE_OF_CONDUCT.md based on Contributor Covenant v2.1
• Reference it from CONTRIBUTING.md

Acceptance Criteria

• Code of Conduct is visible in the repo root
• Community standards section on GitHub shows it