fix(core): cache kill promise to prevent orphan detached processes

cleanUpUnneededContinuousTasks() fires kill() without await. With the
boolean `killing` flag, cleanup() at the end of run() would call kill()
again but no-op (flag already set), leaving the async kill unwaited.
Detached children survive the parent's exit.

Replace the boolean with a cached killPromise. Concurrent callers
(cleanup after fire-and-forget kill) now await the same in-progress
operation. [Self-Healing CI Rerun]

Author: nx-cloud[bot]