logFileFromEnv function
The FA_LOG_FILE env twin of the --log-file flag: the tee path, for
Docker/headless hosts that cannot pass flags. Absent or blank yields
null (no tee intent); the flag wins when both are set.
Implementation
String? logFileFromEnv(Map<String, String> env) {
final path = env['FA_LOG_FILE'];
if (path == null || path.trim().isEmpty) return null;
return path;
}