check

var ErrBehind = errors.NewSentinel(
	"skillup.version_behind",
	"a segment's version is behind what its commits warrant",
)

var ErrInconsistent = errors.NewSentinel(
	"skillup.catalogue_inconsistent",
	"marketplace.json disagrees with the segments on disk",
)

ErrBehind reports at least one segment whose manifest is behind its commits.

NewSentinel, not New: a package-level var captures its stack at package initialisation, which points at nothing useful. A sentinel carries a stable kind instead, so its identity survives a wire. ErrInconsistent reports the catalogue and the segments on disk disagreeing.