Add contributor code of conduct

CODE_OF_CONDUCT.md

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+# Contributor Covenant 3.0 Code of Conduct
+
+## Our Pledge
+
+We pledge to make participation in this project welcoming, safe, and equitable
+for everyone.
+
+We are committed to respecting the dignity, rights, and contributions of all
+participants, regardless of characteristics including race, ethnicity, caste,
+color, age, physical characteristics, neurodiversity, disability, sex or
+gender, gender identity or expression, sexual orientation, language,
+philosophy or religion, national or social origin, socioeconomic position,
+level of education, or other status.
+
+## Encouraged Behaviors
+
+We agree to:
+
+- engage kindly, honestly, and respectfully;
+- respect different viewpoints and experiences;
+- give and accept constructive feedback;
+- take responsibility for our actions and contributions;
+- acknowledge mistakes, apologize when appropriate, and repair harm; and
+- focus on the well-being and technical goals of the project community.
+
+## Restricted Behaviors
+
+The following behaviors are unacceptable:
+
+- harassment or unwanted personal attention after a clear request to stop;
+- insulting, demeaning, discriminatory, or stereotyped comments;
+- sexualized language, imagery, attention, or advances;
+- threats of violence or encouragement of harm;
+- publishing or acting on another person's private information without
+  permission;
+- impersonation or attempts to evade moderation;
+- sustained disruption, trolling, or personal or political attacks; and
+- other conduct that could reasonably threaten the safety or well-being of the
+  community.
+
+## Reporting an Issue
+
+To report a possible violation privately, contact the repository maintainer
+using a private contact method listed on the maintainer's GitHub profile. Do
+not include incident details in a public issue.
+
+If no private contact method is available, open an issue containing only a
+request for a private contact channel. Do not identify the people involved or
+describe the incident in that issue.
+
+Behavior or content hosted on GitHub may also be reported using
+[GitHub's abuse-reporting tools](https://docs.github.com/en/communities/maintaining-your-safety-on-github/reporting-abuse-or-spam).
+
+Reports will be reviewed promptly and handled as confidentially as reasonably
+possible. A maintainer involved in a report must not participate in deciding
+its outcome.
+
+## Enforcement
+
+Project maintainers may remove, edit, or reject comments, commits, code,
+issues, pull requests, and other contributions that violate this Code of
+Conduct. Depending on severity and context, responses may include a private
+warning, temporary interaction restrictions, temporary suspension, or a
+permanent ban.
+
+Enforcement decisions should prioritize community safety, proportionality,
+confidentiality, and opportunities to repair harm when appropriate.
+
+## Scope
+
+This Code of Conduct applies in all project spaces and when an individual is
+officially representing the project in public or other spaces.
+
+## Attribution
+
+This Code of Conduct is adapted from the
+[Contributor Covenant, version 3.0](https://www.contributor-covenant.org/version/3/0/).
+
+Contributor Covenant is stewarded by the Organization for Ethical Source and
+licensed under
+[CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).