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

[krokoko]

Summary

• Adds a new Codeowner input field to the RFC issue template, prompting submitters to identify the GitHub username or team responsible for the affected code area
• Includes a direct link to the repository's CODEOWNERS file for easy reference
• Adds a Codeowner: metadata entry in the admin section at the bottom of the template, alongside the existing Approved by and Reviewed by fields

Build & test results

mise run build — ✅ passed (lint:md, lint:manifests, lint:cross-refs, fmt:check, validate:refs, validate:size, security scans all clean)

Decisions made

• Placed the new codeowner input field immediately after the existing relation field (near the top of the form) so submitters are prompted early to identify ownership before filling out the detailed design sections
• Field is optional (required: false is the default for input type in GitHub issue forms) to avoid blocking submissions where ownership is unclear
• Added a complementary * Codeowner: '' entry in the metadata markdown block to keep admin metadata consistent

Agent notes

What went well: The change was straightforward — the RFC YAML template is well-structured and easy to extend. The build tooling (mise run build) validated everything cleanly on the first run.

What was difficult: Nothing significant; the task was well-scoped.

Patterns/conventions discovered:

• Issue templates live in .github/ISSUE_TEMPLATE/ as .yml files using GitHub's issue forms syntax
• The bottom metadata markdown block in the RFC template follows the pattern * Field: '' for admin-only fields that are intentionally left blank by submitters
• mise run build is the one-stop command that runs all lints, format checks, and security scans — always run this before committing

Suggestions for future tasks:

• Consider making the codeowner field a dropdown once GitHub issue forms support dynamic data, or add a placeholder with example team names from the CODEOWNERS file
• The orphaned .drawio files flagged by validate:refs are pre-existing and not related to this change; they may warrant a follow-up cleanup task

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