Add CODE_OF_CONDUCT.md

[nojibe]

Problem

CONTRIBUTING.md states "We follow standard open source community norms" but there is no actual CODE_OF_CONDUCT.md. GitHub's community health checklist flags this, and contributors have no documented enforcement or reporting path.

Proposed change

Adopt the Contributor Covenant v2.1 as CODE_OF_CONDUCT.md at the repo root, with a real enforcement contact email filled in.

Acceptance criteria

• CODE_OF_CONDUCT.md exists at repo root
• Enforcement/reporting contact is a real, monitored address
• CONTRIBUTING.md links to it instead of describing "standard norms"