feat(storage): emit state_change events on task transitions

UpdateTaskState now writes a state_change event in the same transaction
as the row update, so the event stream and task state never diverge.
Adds UpdateTaskStateBy(id, newState, actor) for explicit actor
attribution; UpdateTaskState delegates with ActorSystem (correct for the
executor, which drives the state machine). RejectTask and
ResetTaskForRetry also emit state_change events attributed to the user.

API accept and cancel handlers now attribute their transitions to the
user via UpdateTaskStateBy. The executor's Store interface is unchanged,
so no test doubles needed updating.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

0 files changed, 0 insertions, 0 deletions