Claude Skills by imbue-ai

Registered a new shared pytest marker, `witnesses(coordinate, partial=None)`, in the centralized marker list. Tests anywhere in the monorepo can now declare which behavior unit they verify (by its coordinate, e.g. `browser-authorization.fresh-code`), with `partial=` noting what the test does not cover. See the `behaviors` skill for the coordinate system.

Registered the new `behaviors` plugin (`imbue-mngr-behaviors`, corpus-generic behavior tooling; see the `mngr_behaviors` changelog entry) in the plugin catalog at the INDEPENDENT tier, listed it as unpublished for now, and added the generated CLI docs page for the `mngr behaviors` command group.