Cancels this run when it fires, or absent when nothing can. Carried here rather than threaded alongside the config because it is per-run input like everything else in state — and it is what lets a verb reach it from the resolved config instead of re-reading the raw arguments.

Only carried: Config.setup never subscribes. Whoever wires a listener has to unwire it, and setup has no teardown, so a shared controller would accumulate one listener per run.