fix(review): scope discard rules gate on cause, not anchored file (trail finding)

The absolute discard order ('findings that point at files not listed are out of scope') dropped a legitimate finding class: an in-scope change that breaks an UNCHANGED caller or consumer in an unlisted file — a cross-file regression, exactly what reviews exist to surface — got discarded because the finding anchors at the broken call site. Both the worker prompt's rule and the judge's scope gate now gate on where the CAUSE lives and explicitly keep in-scope-cause/out-of-scope-impact findings, anchored to the in-scope change.