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

[krokoko]

Summary

• Adds a new optional Codeowner file input field to .github/ISSUE_TEMPLATE/rfc.yml
• The field prompts RFC authors to provide their proposed CODEOWNERS entry (e.g., plugins/my-plugin @awslabs/agent-plugins-my-team) when submitting a new plugin or major feature RFC
• This makes it easier for maintainers to onboard new plugin teams: the CODEOWNERS entry is captured upfront in the RFC rather than discovered later during PR review

Changes

• .github/ISSUE_TEMPLATE/rfc.yml — added codeowner input field (optional, placed after "Alternative solutions")

Build & test results

mise run build   ✅ PASS
  lint:md         0 errors
  lint:manifests  all schemas valid
  lint:cross-refs 0 errors / 0 warnings
  fmt:check       clean
  security:bandit no issues
  security:semgrep 2 pre-existing findings (dependabot cooldown + merge-prevention shell injection) — not introduced by this PR
  security:gitleaks no leaks
  security:checkov 71 passed, 0 failed
  security:grype  no vulnerabilities

No project tests exist beyond the build pipeline; the YAML template was validated via the build.

Decisions

• Field is optional (required: false) — existing RFCs are not affected and not all RFCs require a new plugin team
• Placed after "Alternative solutions" and before the admin metadata section, which is the natural end of submitter-facing questions
• Used type: input (single line) because a CODEOWNERS entry is a short, structured string

Agent notes

What went well: The RFC template and CODEOWNERS file were easy to locate; the change was straightforward with clear precedent from the existing CODEOWNERS structure.

What was difficult: Nothing significant — the YAML template format and existing admin metadata section made the insertion point obvious.

Patterns discovered:

• GitHub issue templates live in .github/ISSUE_TEMPLATE/ as .yml files
• CODEOWNERS has a three-tier model: admins → maintainers → plugin-specific teams
• mise run build is the single command that covers lint + format check + security — always run it before committing

Suggestions for future tasks:

• When adding new plugins, the MAINTAINERS_GUIDE.md already documents the CODEOWNERS entry pattern; link to it from the RFC template description for extra clarity
• Consider making the codeowner field required: true for RFCs with the new-plugin label (not possible in GitHub's issue template YAML, but could be enforced via a GitHub Action)

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