freshen skipped the write whenever no file was added, changed or
removed, so --sign-key and --include-timestamps on an unchanged tree
did nothing. It now skips only when the manifest would come out the
same: nothing in the tree changed, the manifest is signed by the
--sign-key key (unsigned without one), and it has a createdAt
timestamp exactly when --include-timestamps asks for one. A file whose
recorded mode is not what --include-permissions asks for already
counts as changed.
Deviation: the mfer package gains SecretKeyFingerprint and a loaded
manifest's CreatedAt, which the comparison needs.
Model: opus-5-5