chore(github): add codeowner field to RFC issue template

[krokoko]

Summary

• Added a Codeowner metadata field to the RFC issue template (.github/ISSUE_TEMPLATE/rfc.yml) alongside the existing RFC PR, Approved by, and Reviewed by admin fields
• Updated README.md Contributing section to mention the RFC template and its new Codeowner field

Changes

.github/ISSUE_TEMPLATE/rfc.yml

• Added * Codeowner: '' to the bottom admin metadata block

README.md

• Added a paragraph in the Contributing section pointing contributors to the RFC template and explaining the Codeowner field's purpose

Build & Test Results

• mise run lint:md: ✅ PASSED
• mise run fmt:check: ✅ PASSED
• mise run build: ✅ PASSED — all linters, manifest validators, security scans, and format checks passed
• No automated test suite for issue templates or README prose; validated via build pipeline

Agent notes

What went well:

• The change was minimal and targeted — one line in the RFC template and two sentences in the README
• The Contributing section was the natural place to document the RFC workflow and the new field

What was challenging:

• None — very straightforward

Patterns discovered:

• Issue templates use GitHub Forms YAML format (.yml in .github/ISSUE_TEMPLATE/)
• mise run build is the all-in-one validation gate; lint:md and fmt:check can be run individually for faster iteration
• The README is the primary entry point for contributors — the Contributing section is the right place for RFC guidance

Suggestions for future tasks:

• Consider adding a labels field to auto-assign plugin-area labels when an RFC is submitted, matching CODEOWNERS entries
• The RFC metadata section could be formalized as actual form input fields if admin tracking needs become more structured

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.