feat(FN-834): add per-workflow-step model overrides for execution

- Extend WorkflowStep type with optional modelProvider, modelId, validatorModelProvider, validatorModelId fields
- Add API endpoints (PATCH /api/workflow-steps/:id) to update model overrides on workflow steps
- Update executor to use per-step model overrides when running workflow step agents
- Add AgentDetailView support for displaying workflow step model configuration
- Add store and executor tests for new model override behavior

packages/core/src/store.ts

@@ -2529,6 +2529,8 @@ ${stepsSection}`;
         description: input.description,
         prompt: input.prompt || "",
         enabled: input.enabled !== undefined ? input.enabled : true,
+        modelProvider: input.modelProvider,
+        modelId: input.modelId,
         createdAt: now,
         updatedAt: now,
       };
@@ -2579,6 +2581,8 @@ ${stepsSection}`;
       if (updates.description !== undefined) step.description = updates.description;
       if (updates.prompt !== undefined) step.prompt = updates.prompt;
       if (updates.enabled !== undefined) step.enabled = updates.enabled;
+      if ("modelProvider" in updates) step.modelProvider = updates.modelProvider;
+      if ("modelId" in updates) step.modelId = updates.modelId;
       step.updatedAt = new Date().toISOString();
 
       config.workflowSteps = steps;