Function run_session 

pub async fn run_session(
    backend: &dyn AgentBackend,
    spec: SessionSpec,
    log: &mut EventLog,
    paths: &MissionPaths,
    run_meta: RunMeta,
    cancel: Option<Arc<Notify>>,
) -> Result<RunOutcome>

Spawn one session and stream it to completion.

Emits worker.spawned before the session starts and worker.completed after it ends. Result events are aggregated: usage is summed across all of them (streaming sessions emit one per turn), text/is_error come from the last, and cost is the last one reported.

Result mapping: Fail if the last result is_error or the session exit is Failed; Partial if the exit is Aborted (budget/interrupt — forced even when a report parses); otherwise the role’s report decides (Pass only downgradeable by the report; a missing/unparseable report for a worker or validator is Partial, plan §4.6).

cancel: when the notify fires the session is aborted (exit: Aborted).

This is the LogTarget::Live convenience form: the caller passes the shared single-writer log and every event kind is appended to it as it arrives. run_session_to is the same logic over an arbitrary LogTarget, used by the buffered concurrent path (roadmap M3).