policy-engine Python bindings: without bundled-dispatchers, manifests declaring annotators are rejected at construction #3944

The production wheel is built without the bundled-dispatchers feature; AgentControl now rejects any manifest that declares annotators unless annotator_dispatcher is passed. Undocumented in BREAKING_CHANGES.md. Document it or restore a default dispatcher.

Found in the group review of the policy-engine retarget (#3939) and deferred from the follow-up (#3940).