feat(cost-benefit/workflow-cost): fix includeCompleted default to false per api-surface.md spec

The workflowCost function had includeCompleted defaulting to true, but
the api-surface.md specifies the default should be false. Fixed the
default and updated test suite to verify the correct default behavior
and add explicit test for includeCompleted: true opt-in case.

All 562 tests passing across 12 test files.

src/analysis/cost-benefit.ts

@@ -160,7 +160,7 @@ export function workflowCost(
 ): WorkflowCostResult {
   const propagationMode = options?.propagationMode ?? "dag-propagate";
   const defaultQualityRetention = options?.defaultQualityRetention ?? 0.9;
-  const includeCompleted = options?.includeCompleted ?? true;
+  const includeCompleted = options?.includeCompleted ?? false;
 
   // Get topological order — throws CircularDependencyError if cyclic
   const topoOrder = topologicalOrder(graph);