fix(confidence-engine): supply synthesis output schema

lib/graph/current-understanding-synthesis.js

@@ -186,6 +186,15 @@ export const synthesisResponseSchema = z.object({
     .min(1, "currentUnderstanding must be a non-empty string"),
 });
 
+export const synthesisOutputSchema = {
+  type: "object",
+  properties: {
+    currentUnderstanding: { type: "string" },
+  },
+  required: ["currentUnderstanding"],
+  additionalProperties: false,
+};
+
 /** Validate raw provider output against synthesis response schema */
 export function validateSynthesisResponse(raw) {
   if (raw == null) {
@@ -274,7 +283,8 @@ export async function synthesizeCurrentUnderstanding(
   try {
     rawResponse = await provider.generateReconstruction(
       prompt,
-      modelName
+      modelName,
+      synthesisOutputSchema,
     );
   } catch (error) {
     const err = new Error(error.message ?? "Synthesis provider call failed");