Feature: Local multi-agent group conversation (shared room across agents)

### Problem or use case

Users want a **local** multi-agent group conversation: several agents in one shared room/thread so they can discuss or hand work off without going through an external chat product.

### Proposed behavior (sketch)
- A first-class “group / room” surface where multiple configured agents can participate in one conversation.
- Local-only path (no requirement on a third-party hosted multi-agent product).
- Clear ownership of which agent responds, history, and how work is delegated.

### Related (not the same request)
- #12538 — project groups as a first-class agent surface (group-level instructions/memory/chat for a *project group*). Overlaps on “group + agents” but is not the same as an multi-agent chat room UX.

### Environment ask
Please comment with preferred agents, whether this should be desktop-only vs mobile, and any must-have UX from other tools.

Start by reviewing this issue alongside related issue #12538 to separate a shared chat room from project groups. The payload names no files, tests, or entry points and leaves ownership, history, delegation, platform scope, and must-have UX unresolved; done requires those design decisions and an agreed implementation scope.

Written by the indexing model from the issue text.