Enable/disable a subcontractor here without detaching it.

The engine has always gated on this flag — pipeline_context_cache filters is_enabled.is_(True) when it collects the sourced operation kinds — but no route set it, so it could never be anything but the True it defaults to on attach. A shop wanting to stop sourcing one operation from one subcontractor in one environment had to detach and re-attach, discarding the SCD2 history the membership exists to keep. Same shape as the tool sibling above.