Use Go 1.13+ octal literal syntax throughout codebase

Update file permission literals from legacy octal format (0755, 0644)
to explicit Go 1.13+ format (0o755, 0o644) for improved readability.

internal/cli/fetch.go

@@ -257,7 +257,7 @@ func downloadFile(fileURL, localPath string, entry *mfer.MFFilePath, progress ch
 	// Create parent directories if needed
 	dir := filepath.Dir(localPath)
 	if dir != "" && dir != "." {
-		if err := os.MkdirAll(dir, 0755); err != nil {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
 			return err
 		}
 	}