ANU-6: Add the containerized local CMS development stack

This directory records the repository-owned implementation and verification for ANU-6, the first CMS child of Product Main change 2026-07-20-switch-to-local-postgres.

• Implementation plan
• Implementation log
• Completion report

The change adds only the repeatable local development stack and guarded migration tooling. It does not export, restore, or cut over the real Supabase database; that stateful operation belongs to the dependent migration issue.