`Canceller::cancel()` does nothing and returns `false` when no ACP turn
is in flight: from `start` until the run's task begins its turn, and
from a turn's end until `after_turn` releases the name. `interrupt`
ignored that return, recorded `Cancelled` and replied that the run was
cancelled, so a non-goal run went on to do its whole first turn while
`status` read as cancelled.
On ACP, `interrupt` now records the stop, cancels, and if nothing was
in flight withdraws the stop under the same `stops` lock, restores the
tracking entry, and refuses with the claude path's "still starting, try
again shortly". The claude arm is unchanged.
Review finding on #4824 (argus).
3bef1dfab6