Close orchestrator before deleting task cleanup test root

[conornash]

This one doesn't seem to be Mac-specific. I occasionally hit a failure in test-task-child-cancels-lisp-executions where cleanup couldn't delete its temporary directory. I believe it's a timing issue in the test rather than in Autolith itself, but I may well have missed something, so please do correct me.

The below is the agent explanation of what this PR does:

test-task-child-cancels-lisp-executions failed intermittently with "Could not delete directory". The aborted execution becomes terminal, waking session-job-await, before its thread runs pool listeners; the completion subscription then publishes data/tasks/<id>/completion-*.sexp while the test deletes its root. The test's orchestrator comes from task-augment-tool-registry, so the suite-level orchestrator cleanup never closed it.

The cleanup now calls task-orchestrator-close, which joins both pools before the root is deleted. In a 15-iteration loop: 6 delete failures before, 0 after; the task-execution suite passes.