A delegate_task fan-out batch occupies ONE async-pool slot by design, so
active_count() (unit/slot count) reports a 3-task batch as 1 — which
undercounts real concurrent subagent load on the background_work metric.
Add active_task_count() that expands a running batch to its child count
(N-task batch -> N, single -> 1) and switch the background_work reader to
it. active_count() is unchanged (capacity semantics preserved). Adds a
contract test for the unit-vs-task distinction and documents both counts.
Victor Kyriazakos
0ef0816284