ReadonlytimestampISO-8601 timestamp.
ReadonlykindEvent kind prefix (transition, tool_call, error, lifecycle).
ReadonlyeventFull event name (e.g., "transition:PLAN_READY").
ReadonlyphasePhase at this point.
ReadonlyactorActor (machine, human, system).
ReadonlydescriptionOne-line human-readable description.
A single entry in the session timeline.