fix(confidence-engine): supply overview-specific output schema to generateReconstruction

synthesizeInvestigationOverview() called provider.generateReconstruction(prompt, modelName)
without an output contract. Without a structured-output schema the provider defaults to its
initial-reconstruction format which lacks { understanding, plausibleInterpretations }, causing
'Missing or invalid required fields' validation failure.

Add overviewOutputSchema (JSON-schema matching validateOverviewResponse contract) and pass as
third argument, matching the proven pattern in synthesizeCurrentUnderstanding().

53/53 tests PASS. Co-Authored-By: Claude Code <noreply@anthropic.com>

docs/current-handoff.md

@@ -391,6 +391,7 @@ RAW USER EVIDENCE
 
 - `synthesizeInvestigationOverview()` previously passed the full provider envelope (`{ response: { understanding, plausibleInterpretations }, providerApiPath, providerExecution }`) directly into `validateOverviewResponse()`, which expected only `{ understanding, plausibleInterpretations }`. This caused "Missing or invalid required fields" on real providers.
 - Fix now unwraps `.response` before overview validation, matching the proven pattern in `lib/graph/current-understanding-synthesis.js` (line 296: `validateSynthesisResponse(providerResult.response)`).
+- **Additional fix:** `synthesizeInvestigationOverview()` previously called `provider.generateReconstruction(prompt, modelName)` without an output schema, while `synthesizeCurrentUnderstanding()` passes `synthesisOutputSchema`. Without a structured-output contract the provider defaults to initial-reconstruction format, which lacks `{ understanding, plausibleInterpretations }`, causing validation failure. Fix adds `overviewOutputSchema` (JSON-schema: `{ understanding: string, plausibleInterpretations: string }`) and passes it as third argument.
 - Owning regression (`tests/graph/investigation-overview-synthesis.test.js`) now mirrors real provider envelope shape (`{ response: { understanding, plausibleInterpretations }, providerApiPath }`). Regression fails against pre-fix seam and passes after fix.
 - Canonical overview live helper `scripts/overview-experiment-helper.cjs` established with deterministic no-network self-test (zero live calls). Product-launch zero-Findings fixture reaches successful overview synthesis in deterministic mode. 53/53 overview tests pass.
 - **Zero live proof yet.** Zero live calls made in this handoff.

lib/graph/investigation-overview-synthesis.js

@@ -31,6 +31,16 @@ const overviewResponseSchema = z.object({
   plausibleInterpretations: z.string().min(1),
 });
 
+const overviewOutputSchema = {
+  type: "object",
+  properties: {
+    understanding: { type: "string" },
+    plausibleInterpretations: { type: "string" },
+  },
+  required: ["understanding", "plausibleInterpretations"],
+  additionalProperties: false,
+};
+
 const FORBIDDEN_FIELD_NAMES = new Set([
   "recommendation",
   "decision",
@@ -245,6 +255,7 @@ export async function synthesizeInvestigationOverview(params, deps) {
     providerResult = await provider.generateReconstruction(
       prompt,
       modelName,
+      overviewOutputSchema,
     );
   } catch (err) {
     throw new Error(`Overview synthesis provider call failed: ${err.message}`);