Add a short maintainer checklist for reviewing first-time PRs

[Permea-lab-admin]

Goal

Make it easier for maintainers to review first-time contributor PRs consistently.

Suggested scope

• Add a concise checklist to docs/community/MAINTAINER_GUIDE.md or a nearby community doc.
• Include checks for scope, claim boundaries, link validity, and contributor tone.
• Keep it practical and short.

Acceptance criteria

• The checklist has 5-8 actionable items.
• It complements existing contributor and reviewer docs.
• It does not introduce heavy process for small PRs.

Contributor note

This is a documentation task suitable for contributors interested in OSS maintenance.