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

[krokoko]

Summary

• Adds a new optional Codeowner input field to the RFC issue template (.github/ISSUE_TEMPLATE/rfc.yml)
• The field prompts RFC submitters to provide the GitHub team that will own the proposed plugin/feature (e.g. @awslabs/agent-plugins-<team-name>)
• Field is optional to keep the barrier for submission low while capturing ownership intent early

Decisions

The field was placed after "Alternative solutions" (the last optional field) and before the admin metadata footer — keeping required fields at the top and optional governance fields at the bottom. It is marked required: false since not all RFCs will have a clear team owner at proposal time.

Build & Test Results

mise run build  →  PASS
  lint:md           ✓  0 errors
  lint:manifests    ✓  all JSON schemas valid
  lint:cross-refs   ✓  0 errors / 0 warnings
  fmt:check         ✓  no formatting issues
  validate:refs     ✓  0 broken links
  validate:size     ✓  all skills within limits
  security:bandit   ✓  no issues
  security:semgrep  ✓  2 pre-existing findings (unrelated)
  security:gitleaks ✓  no leaks
  security:checkov  ✓  71 passed / 0 failed
  security:grype    ✓  no vulnerabilities

Agent notes

What went well:

• The task was narrowly scoped — one YAML file to edit with a clear pattern to follow from the existing fields.
• The CODEOWNERS instructions file (codeowners.instructions.md) gave clear guidance on the expected team name format, which I used for the placeholder and description.

What was difficult:

• Nothing particularly difficult; the existing template fields provided ample structure to model the new field after.

Patterns discovered:

• Issue templates live in .github/ISSUE_TEMPLATE/*.yml as standard GitHub form schema.
• The CODEOWNERS file has a companion instructions file at .github/instructions/codeowners.instructions.md that documents formatting rules for maintainers.
• Teams follow the pattern @awslabs/agent-plugins-<plugin-name>.

Suggestions for future tasks:

• If more governance fields are needed (e.g. "Milestone", "Estimated effort"), they should also go in the optional section after the core content fields.
• Consider adding a dropdown field for plugin category to enable easier triage filtering.

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