Governance: CODEOWNERS and RFC template for protocol changes

Goal: Establish a lightweight governance process for protocol evolution.\n\nScope\n- Add CODEOWNERS for review routing\n- Add an RFC issue/PR template (proposal, discussion, decision)\n- Document contribution guidelines for protocol changes\n\nAcceptance Criteria\n- CODEOWNERS present and enforced\n- .github/ISSUE_TEMPLATE/RFC.md or PR template added\n- CONTRIBUTING.md updated with the process