docs: add Contributor Covenant CODE_OF_CONDUCT.md

[cceebb]

Summary

Added a CODE_OF_CONDUCT.md file using the Contributor Covenant v2.1 template to establish clear community guidelines and expected contributor behavior.

Type of Change

• New agent example
• Bug fix
• Documentation update
• Refactor / cleanup
• Other

Checklist

• I have starred this repository.
• New community agents are under contributors/<agent-name>/ (not repo root).
• I ran ruff check ..
• I ran ruff format ..
• I added/updated README.md for changed example(s).
• I added .env.example if environment variables are required.
• I added demo image/GIF (if applicable).
• I added agent profile link (if applicable).
• I updated contributors/CHANGELOG.md for community agent changes, or root CHANGELOG.md for other non-doc changes.
• I added my agent to the Community Contributors table in root README.md (if new agent).
• I verified paths/commands used in docs.
• I understand this PR requires maintainer review before merge (review-required CI).

Related Issue

Closes #161

Notes for Reviewers

Added a standard Contributor Covenant v2.1 Code of Conduct at the repository root.