Deletes one step on the workflow draft and relinks linear successors to keep
the chain intact. Rejects deletes when nested child steps still reference the
target as their parent — delete children first (via this endpoint).

Validation

• Ensures the workflow and step exist on the editable draft for the
  authenticated organization.
• Re-validates graph placement after the delete + relink.
• Uses optimistic concurrency against the draft (or published version when
  creating a draft). Concurrent races return 409 stale_draft.

Returns the deleted step payload. OAuth callers require the
workflows:update scope (and matching workspace permission). API key
callers are authorized by organization membership.