The corpus comes first Shipped
narrate consumes a store it does not build. A separate gathering step has to run first, or there is nothing to be grounded in.
The gathering step walks git history, documentation, memories and cross-references into a store; narrate only reads it. Splitting them is what makes the writing grounded rather than recalled — the story is assembled from a corpus you can inspect, and a claim with no corpus behind it has nowhere to come from.
The failure mode it prevents
A narrative model with no corpus does not refuse — it writes something plausible. Requiring the store to exist first turns "the model made that up" into "the gathering step did not run," which is a diagnosable problem rather than a subtly wrong document.