Chroma: document persistence, reset, and Postgres vs vector parity

[rosspeili]

Problem

docs/TROUBLESHOOTING.md mentions desync when “Chroma was wiped”; room.py uses per-room collections (docs/ARCHITECTURE.md). Operators lack a single “data lifecycle” story (backup, delete, GDPR-style scrub).

Goal

Extend docs/MEMORY_AND_RAG.md (or new short section) with: where Chroma stores data in this build, what happens on empty collection, room collection naming, and recommended backup/rotation.

Acceptance criteria

• Doc matches actual chromadb.Client usage and collection names.
• Cross-link from README or troubleshooting for “Chroma desync.”