Create a workbench (so a workflow has somewhere to write)

Create an empty workbench and return its workbench_id.

The missing half of the loop: an agent could be TOLD a workbench id but could never make one, so "build me a workflow that collects X into a new workbench" always needed a human to go and click New Workbench first.

entity_type is optional and must be contact or company when given; a blank workbench may defer it, which is what the underlying command already allows. created_by is the authenticated caller and is never read from the request, so a key cannot attribute a workbench to someone else.

The webhook API key the browser route returns on create is deliberately NOT surfaced here. It is a write credential for a different ingestion path, it is returned exactly once, and nothing in the workflow surface needs it — an agent that never sees it cannot leak it into a workflow config or a chat transcript.