Docs/contributor governance

[D-Ochuko]

Summary

This PR adds the missing contributor-facing governance and onboarding documentation needed to make the repository usable for external contributors and maintainers.

It includes:

• a real root CONTRIBUTING.md covering prerequisites, local setup, Prisma workflow, lint/typecheck/test commands, commit conventions, and the expected PR flow
• a repository CODE_OF_CONDUCT.md with a clear reporting path and enforcement ladder
• a root CODEOWNERS file mapping review ownership for the major backend subsystems
• a lightweight RFC process under docs/rfcs/ for protocol-affecting changes

This is intentionally a narrow documentation-only change focused on contributor workflow, governance, and design review process.

Changes

• Added contributor setup and workflow guidance to CONTRIBUTING.md
• Linked contributor onboarding documentation to the project README
• Added a standard Contributor Covenant-style code of conduct with reporting and escalation guidance
• Added review-routing ownership via CODEOWNERS
• Added RFC template and process docs for changes touching protocol, persistence, or WS semantics

Testing

• Ran npm run lint
• Repo lint output shows only existing warnings and no new lint errors from this documentation patch

Related issues

closes #307
closes #308
closes #310
closes #311

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