fix(review): persist findings even when finalize is cancelled

When the user Ctrl+C's out of a slow final-report/synthesis step, the run context is cancelled AFTER summary.Cancelled was computed for already- finished workers. The summary.Cancelled guard therefore passes, but the git/disk work in writePostReviewManifest then ran on the cancelled context and failed with "write to disk failed: resolve git common dir: git rev-parse: context canceled" — silently discarding findings the user waited minutes for.

Detach persistence from the run context (context.WithoutCancel) with its own 30s deadline so completed findings survive a cancel during finalize. A genuinely cancelled run still skips via the existing summary.Cancelled guard.