[SandObserver]

Summary

• Add CODE_OF_CONDUCT.md: Contributor Covenant 2.1, taken verbatim from the upstream source with only the [INSERT CONTACT METHOD] placeholder replaced by the project's reporting address.
• Link it from the "More" list in CONTRIBUTING.md and from the contributing paragraph in README.md.

Testing
All green. No new tests: documentation only. The file was diffed against the upstream 2.1 text to confirm the contact line is the sole difference.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.