fix(senpi-task): keep the caller's subagent_type in the status target

formatTargetIdentity preferred the category whenever a record carried both
identities, erasing the subagent_type the caller actually wrote; such a row claimed
a category the caller never asked for. Both are now rendered as
agent:<asked>->category:<used>, so the model and the target that selected it always
travel together. The task tool schema and description now state that a category name
is not accepted as a subagent_type.

Refs #8348