Fork a conversation: a NEW session seeded with this one's full accumulated context (G-MSESS, Misha's actual ask — 'run two parallel sessions' meant taking the context somewhere else; his fresh session B re-derived it for 40 minutes and got worse answers).

Copies the agent_session_entries rows under a freshly minted id — the SDK resumes from OUR store, so any id with rows is a resumable session. Because the store only ever holds COMPLETED turns, branching a running conversation is the same operation as branching an idle one: the branch knows everything up to the last finished answer; the in-flight turn's result stays with the parent. No locks, no racing the live turn.