feat(sync): add incremental export for large repos

For repos with 1000+ issues where less than 20% are dirty,
incremental export reads the existing JSONL, merges only
changed issues, and writes back - avoiding full re-export.

- Add exportToJSONLIncrementalDeferred as new default export path
- Add shouldUseIncrementalExport to check thresholds
- Add performIncrementalExport for merge-based export
- Add readJSONLToMap helper for fast JSONL parsing
- Falls back to full export when incremental is not beneficial

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync.go

@@ -772,8 +772,8 @@ func doExportSync(ctx context.Context, jsonlPath string, force, dryRun bool) err
 			}
 		}
 
-		// Export to JSONL
-		result, err := exportToJSONLDeferred(ctx, jsonlPath)
+		// Export to JSONL (uses incremental export for large repos)
+		result, err := exportToJSONLIncrementalDeferred(ctx, jsonlPath)
 		if err != nil {
 			return fmt.Errorf("exporting: %w", err)
 		}