Cancel Agent Run

Stop a run and everything it spawned.

OWNER ONLY, and that is stricter than the read routes on purpose: shared visibility lets an org SEE a teammate's run, while stopping it is destructive and not undone by restarting. Children are signalled before the root so a freed parent cannot leave orphans behind it.

Idempotent on an already-terminal run: cancelled: false with a reason, not an error, so a double call does not look like a failure.