Adds mr_diffs_fetched and mr_diffs_failed fields to IngestResult and
SyncResult, threads them through the orchestrator aggregation, includes
them in the structured tracing span and human-readable sync summary.
Previously MR diff failures were silently swallowed — now they appear
alongside resource event counts for full pipeline observability.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>