feat: add room persistence scoped to a workspace

[ibourgeois]

Summary

Add rooms as shared workspace conversation containers separate from private chats.

Why This Matters

Rooms are where shared team context, workflows, and role-based features will live. They need a distinct persistence model from private chats.

Acceptance Criteria

• add a room model that belongs to a workspace
• add durable room conversation persistence separate from private chats
• the shell can list rooms distinctly from private chats
• room conversations are marked as shared workspace context
• room data model leaves room for role- or workflow-specific capabilities later

Notes

This issue is about the room container and shared conversation persistence, not graph indexing or specialized tools yet.