check, fetch, gen and freshen took --require-signature or --sign-key
given an empty value, on the command line or through
MFER_REQUIRE_SIGNATURE or MFER_SIGN_KEY, as the flag not given, so an
unset shell variable skipped the signature check or the signing. Each
now asks whether the flag was given, as gen's --output does. An empty
fingerprint is refused as one of the wrong length, by fetch before it
requests anything; an empty --sign-key is refused, naming the flag,
before any file is read.
Model: opus-5-5
sneak