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

[krokoko]

Summary

• Added a new optional Codeowner file input field to .github/ISSUE_TEMPLATE/rfc.yml
• The field prompts RFC authors to specify the team or path entry that should be added to the CODEOWNERS file for their proposed feature (e.g. @awslabs/agent-plugins-my-plugin-team)
• Field is optional (required: false) to avoid blocking proposals that don't require new ownership entries

Decisions

The field was placed after "Alternative solutions" and before the admin metadata section, keeping it logically grouped with implementation-related metadata. Making it optional ensures it doesn't block RFCs that don't introduce new plugin ownership requirements.

Build & Test Results

• mise run build ✅ passed (lint, manifest validation, cross-refs, fmt check, security scans — all clean)
• No tests beyond the build pipeline exist for issue templates

Agent notes

What went well: The change was straightforward — the RFC template is a simple YAML file and the CODEOWNERS instructions file (codeowners.instructions.md) made the intent of ownership fields very clear.

Patterns discovered: The repo uses GitHub issue form YAML templates (not the older markdown-based templates). All linting is handled via mise run build, which covers markdown, JSON manifests, cross-references, formatting, and security scans in one command.

Suggestions: Future tasks touching .github/ISSUE_TEMPLATE/ can follow the same pattern — add fields using the GitHub Forms YAML spec, run mise run build, and open a PR.

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