shape-doctor.py: a root's own contracts/shape-pin.yaml that does not read as a pin (a merge conflict, a non-UTF-8 byte, a commit: that is not 40 hex, a deleted file) is INVALID, exit 1, with the git command that puts back the last pin a tool wrote as its NEXT, instead of CANNOT ANSWER, exit 3, sending the reader to git fetch --all in the standard (#237) - #267