~anthropic/claude-sonnet-latest → B (8:2)

Side B adds a real end-to-end feature (CLI `room create` command, RPC wiring, docs/guide updates) plus a genuine simplification removing the unused ThreadVisibility concept and its plumbing across events, reducer, timeline, and tests. Side A is a small, correct but minor dead-code cleanup in the reducer with a test tweak, of much smaller scope and impact.