Add defensive review checklist

[itsDNNS]

Summary

• Adds a repo-local defensive review checklist for auth, integrations, exports, backups, shared-device surfaces, and self-hosted changes
• Links the checklist from SECURITY.md and CONTRIBUTING.md
• Records the checklist as repo-local maintainer workflow documentation in the documentation ownership policy

Test Plan

• Markdown local-link check for changed docs
• git diff --check
• Defensive wording scan for internal workflow language and alarmist security terms

Closes #296