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

[krokoko]

Summary

• Added a new optional Codeowner file input field to .github/ISSUE_TEMPLATE/rfc.yml
• The field allows RFC submitters to specify the relevant codeowner path or team (e.g. plugins/deploy-on-aws or @awslabs/agent-plugins-maintainers), helping maintainers route reviews appropriately

Changes

.github/ISSUE_TEMPLATE/rfc.yml — new input block inserted after the existing relation field:

- type: input
  id: codeowner
  attributes:
    label: Codeowner file
    description: Please specify the codeowner file or team responsible for this area
    placeholder: e.g. plugins/my-plugin or @awslabs/agent-plugins-maintainers
  validations:
    required: false

Build and test results

• mise run build — ✅ passed (lint:md, lint:manifests, lint:cross-refs, fmt:check, validate:refs, validate:size, security:bandit, security:semgrep, security:gitleaks, security:checkov, security:grype all green)
• No tests specific to YAML issue templates; validated by the successful build

Agent notes

What went well:

• The change was straightforward — a single new input field added to the YAML issue template
• The full build (mise run build) passed without any issues on the first run

What was difficult:

• Nothing significant; the YAML structure was clear and consistent with the existing field format

Patterns discovered:

• Issue templates live in .github/ISSUE_TEMPLATE/ as .yml files
• The CODEOWNERS file already lists all plugin teams; the RFC field labels mirror those paths
• mise run build covers lint + format check + security scanning in one command

Suggestions for future tasks:

• Consider making the codeowner field pre-populated via a dropdown of known teams once GitHub supports that in issue forms
• A mise run lint:templates task validating YAML issue templates would be a nice addition

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