docs: add code of conduct

[janisto]

Why

The project needs a concise statement of expected participant behavior that also makes contributors accountable for AI-assisted submissions and discourages unreviewed or mass-generated repository noise.

What changed

• add a root CODE_OF_CONDUCT.md
• require respectful, professional, and accountable participation
• prohibit harassment, personal attacks, deliberate disruption, and low-effort or misleading mass submissions
• state the maintainers' moderation authority

Validation

• just qa: passed
• git diff --check: passed
• Additional checks: verified the commit adds only CODE_OF_CONDUCT.md and that its contents are identical across the rollout

Compatibility and risk

• Compatibility impact: None; this is community documentation only.
• Security and privacy impact: None.
• Failure mode and remaining risk: The deliberately short policy provides less procedural detail than a comprehensive code of conduct, but it clearly defines behavior and enforcement expectations.

Documentation and changelog

The code of conduct is the documentation change. No changelog entry is needed because package behavior is unchanged.