Add CODE_OF_CONDUCT.md (open-source readiness)

Part of the go-public readiness work. Add a root CODE_OF_CONDUCT.md — the one standard OSS file missing alongside LICENSE/README/CONTRIBUTING/SECURITY.

Behavioural standards adapted from Contributor Covenant 2.1, matching the existing docs' tone (Discord + Tesks routing, no exposed email). Includes an explicit project governance / final-say section: Rían Errity (owner/lead maintainer) retains final authority over what goes into the plugins — features, scope, accept/reject — and contributors do not unilaterally decide what features are allowed or the project's direction.