No CODE_OF_CONDUCT.md — missing community health standard

[codeforester]

Problem

The repository has no CODE_OF_CONDUCT.md. This is standard for open source projects and is checked by GitHub's community health dashboard. Without it:

• GitHub shows the project as missing a community standard
• Potential contributors have no written expectations for participation
• PyPI's community guidelines recommend linking to a code of conduct

Fix

Add CODE_OF_CONDUCT.md to the repository root adopting the Contributor Covenant 2.1. The contact email field should be filled in.

After adding, update pyproject.toml if there is a URL field for the code of conduct, and verify GitHub community health shows green.