Capture raw task events as a drainable stream.
Surfaces stream_mode="tasks" data on run.tasks as a
StreamChannel[dict[str, Any]]. Each item is a task payload
(start or result).
LifecycleTransformer and SubgraphTransformer also consume
tasks events for subgraph discovery and lifecycle tracking.
This transformer captures the raw payloads independently for
consumers who need task-level detail.
Only events at the run's own scope are captured; task data from
deeper subgraphs is available on the respective subgraph handle's
.tasks projection.
Native transformer — run.tasks is a direct attribute.
