fix: Resolve 11 errcheck linter violations to unblock CI (bd-91)

Fixed all unchecked error returns in production code:
- os.Remove() calls in cleanup paths
- cmd.Wait() in goroutines
- fmt.Fprintf() writes
- Type assertions with proper ok checks

Reduces linter issues from 99 to 88. CI should now pass linting.

cmd/bd/sync.go

@@ -312,8 +312,8 @@ func exportToJSONL(ctx context.Context, jsonlPath string) error {
 	}
 	tempPath := tempFile.Name()
 	defer func() {
-		tempFile.Close()
-		os.Remove(tempPath)
+		_ = tempFile.Close()
+		_ = os.Remove(tempPath)
 	}()
 
 	// Write JSONL