feat: add private chat persistence scoped to a workspace #134
Description
Summary
Add the first durable private chat model beneath a workspace.
Why This Matters
The first user-facing goal is to let someone talk to agents. That needs a real private conversation surface with durable messages before room behavior or graph indexing gets layered on top.
Acceptance Criteria
- add a chat model that belongs to a workspace
- add a message model for chat conversation history
- support private 1:1 and private group chats at the data-model level
- chats are visible only within their parent workspace
- chat conversations are explicitly marked as private and are not written into the shared workspace graph
Notes
This issue is about data model and persistence boundaries, not agent execution yet.