Add CODE_OF_CONDUCT.md

[nora-weisser]

Goal

Add a CODE_OF_CONDUCT.md so the project sets clear community standards for participation.

Scope

• Use Contributor Covenant v2.1.
• Set the WCC community contact email as the reporting channel.
• Link it from README.md and CONTRIBUTING.md.

Acceptance criteria

• CODE_OF_CONDUCT.md exists at repo root (Contributor Covenant v2.1)
• Reporting contact email is filled in
• Linked from README and CONTRIBUTING