rootDescendantCounts was incremented on every spawn but never decremented
when tasks reached terminal states (completed, cancelled, error, interrupt,
stale-pruned). This made maxDescendants=50 a session-lifetime quota instead
of its intended semantics as a concurrent-active agent cap.
Fix: add unregisterRootDescendant() in five terminal-state handlers:
- tryCompleteTask(): task completes successfully
- cancelTask(): running task cancelled (wasRunning guard prevents
double-decrement for pending tasks already handled by
rollbackPreStartDescendantReservation)
- session.error handler: task errors
- promptAsync catch (startTask): task interrupted on launch
- promptAsync catch (resume): task interrupted on resume
- onTaskPruned callback: stale task pruned (wasPending guard)
Fixes: code-yeongyu/oh-my-openagent#2700
Casey Howard
5986583641