Address new Cursor finding on PR #11886 (failed polls skip staleness)

Cursor Bugbot Medium finding ec685985 posted 2026-06-08T13:38:14Z on sftpPoll/index.ts. PR #11886 was already merged before fix could land on that branch. Fix cherry-picked to cato/bolt-1211-staleness-on-failed-polls, PR #11888 opened. Fix: declare target outside try block, evaluate staleness with newFilesDetectedThisPoll=0 in catch block, wrapped in inner try/catch so staleness errors never mask poll errors. All 13 staleness tests green locally.