Post a rendered DAG status message to a configurable Zulip stream/topic
when a Hatchet workflow is dispatched. Update it after each task
completes (success or failure), and delete it when post_zulip runs
so the final transcript notification replaces it.
On pipeline failure, appends an error banner (:cross_mark:) with the
failing step name and error message.
New settings: ZULIP_DAG_STREAM, ZULIP_DAG_TOPIC, ZULIP_HOST_HEADER.
New module: reflector/hatchet/dag_zulip.py (create/update/delete).
