feat(sync): surface MR diff fetch/fail counters in sync output

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>

src/cli/commands/ingest.rs

@@ -42,6 +42,8 @@ pub struct IngestResult {
     pub notes_upserted: usize,
     pub resource_events_fetched: usize,
     pub resource_events_failed: usize,
+    pub mr_diffs_fetched: usize,
+    pub mr_diffs_failed: usize,
 }
 
 #[derive(Debug, Default, Clone, Serialize)]
@@ -606,6 +608,8 @@ async fn run_ingest_inner(
                 total.mrs_skipped_discussion_sync += result.mrs_skipped_discussion_sync;
                 total.resource_events_fetched += result.resource_events_fetched;
                 total.resource_events_failed += result.resource_events_failed;
+                total.mr_diffs_fetched += result.mr_diffs_fetched;
+                total.mr_diffs_failed += result.mr_diffs_failed;
             }
         }
     }