Adds a `debug` boolean to retriever_query that escalates the engine's CloudWatch
log level to DEBUG (logLevels=ERROR,INFO,PERF,DEBUG) for ONE query — per-blob
Bloom decisions, per-fetch S3 reads, per-event results-writer samples — for
0-result/truncated-outcome triage. Plumbs logLevels through RetrieverQueryRequest
into the engine REST body. The diagnostics module itself was already in main
(retriever-diagnostics.ts); this is the remaining unique sliver. Suite 1207 green.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>