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>