feat: index room conversations into the workspace graph

[ibourgeois]

Summary

Write shared room conversation history into the workspace graph while keeping private chats out of that index.

Why This Matters

The product distinction between rooms and chats only becomes meaningful once shared room conversation history contributes to durable workspace context and private chat history does not.

Acceptance Criteria

• room conversations are ingested into the workspace graph or equivalent shared retrieval layer
• private chats are explicitly excluded from that graph ingestion path
• workspace graph queries can reference room conversation context
• the implementation leaves a clean boundary for future GraphRAG and retrieval work

Notes

This issue should integrate with existing graph and retrieval direction, including the broader GraphRAG work tracked elsewhere.