Document the GitHub-native contribution workflow

[Happypig375]

Summary

• document the repository's GitHub-native issue and pull-request workflow
• distinguish core changes that need human review from peripheral changes that may rely on green CI and satisfactory independent LLM review
• add core-only code ownership and repository-local coding-agent guidance
• require concise scope, validation, maintenance cost, actual AI-review disclosure, and explicit human-judgment needs in pull-request descriptions

Related issue

Incorporates the review-policy discussion in #273 and its replies. This does not close #273.

Validation

• git diff --check
• focused policy validation for non-empty files, trailing whitespace, CODEOWNERS owners and paths, peripheral exclusions, and required review language
• GitHub Actions: 6 successful, 4 skipped, 0 failing

Added code and maintenance

No runtime, public-API, rendering, serialization, platform, or migration impact. The new policy and CODEOWNERS map must be updated when repository boundaries or maintainer access change. CODEOWNERS requests reviews for matching core paths; the repository currently has no branch rule that enforces approval.

Review

Independent Codex review found two policy/ownership gaps and one formatting issue; all were fixed, and the final review was approved. GitHub Copilot was not used or assumed.

Human judgment

Yes. A maintainer should confirm the core/periphery boundary, owner list, and whether review enforcement should be configured separately.