bd-162: Add database integrity checks with oracle review fixes

- Added validatePreExport to prevent data loss
- Added checkDuplicateIDs to detect corruption
- Added checkOrphanedDeps to find orphaned dependencies (both sides)
- Added validatePostImport to ensure imports don't lose data
- CRITICAL FIX: Removed post-pull export that clobbered fresh JSONL
- Conservative checks when JSONL is unreadable
- Efficient COUNT(*) SQL path instead of loading all issues
- Comprehensive test coverage including edge cases

cmd/bd/export.go

@@ -34,9 +34,8 @@ func countIssuesInJSONL(path string) (int, error) {
 			if err.Error() == "EOF" {
 				break
 			}
-			// If we hit a decode error, stop counting but return what we have
-			// This handles partially corrupt files
-			break
+			// Return error for corrupt/invalid JSON
+			return count, fmt.Errorf("invalid JSON at issue %d: %w", count+1, err)
 		}
 		count++
 	}