Closes#2831. post_logout's wipe_summary only checked whether cleared.wiped
was empty, so a real .credentials.json blocked by EPERM (nothing made it
into wiped) got reported as 'no credential files present (already logged
out)' in the same breath as a warning saying the delete failed -- self
contradictory and misleading, since the file is still there un-deleted.
Extracted the branch into a pure wipe_summary() helper (wiped file(s) named >
warnings-only failure > genuinely nothing found) with unit tests covering
the exact permission-error shape from the issue.
