fix(ingestion): wrap naked error returns and harden mustJSON helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/wiki/log.go

@@ -33,6 +33,8 @@ func AppendLog(brainDir, source string, pages, warnings []string, date string) e
 		return fmt.Errorf("open log: %w", err)
 	}
 	defer f.Close()
-	_, err = f.WriteString(sb.String())
-	return err
+	if _, err = f.WriteString(sb.String()); err != nil {
+		return fmt.Errorf("write log: %w", err)
+	}
+	return nil
 }