initialize method
Initialize history service and create directories.
Implementation
Future<void> initialize() async {
if (_initialized) return;
final dir = Directory(_baseDir);
if (!await dir.exists()) {
await dir.create(recursive: true);
}
// Start periodic flush (every 5 seconds).
_flushTimer = Timer.periodic(const Duration(seconds: 5), (_) => flush());
// Load session summaries index.
await _loadSummaryIndex();
_initialized = true;
}