Function start_session 

pub async fn start_session(
    state: &Arc<ServerState>,
    args: StartArgs,
    generator: Arc<dyn TurnGenerator>,
) -> Result<Value, String>

Provision worktree + derive contract + register the session. Returns the start response value.

The work runs on a daemon-owned task (ServerState::spawn_durable_operation), not on the caller’s future, and this wrapper only awaits its result. That is load-bearing, not tidiness: coder.start is dispatched on the per-connection conn_tasks JoinSet, which abort_all()s the instant the WebSocket closes. [start_session_inner] registers the session and provisions its worktree before the multi-minute contract derivation, so a board that quit during drafting used to cancel the very run the board had just told the operator would keep going — leaving a drafting session row, a leaked worktree, no contract and no driver until the daemon restarted. A caller that stays connected sees the identical response, at the identical time; a caller that disappears now loses only its own response waiter.