readFile method
Reads a structure/index/graph file by its logical path.
Examples: INDEX.md, GRAPH.md, stats/activity_timeline.md,
topics/state-management.md.
Returns null if the file does not exist.
Implementation
@override
String? readFile(String path) {
final file = File('${_path(kbDir)}/$path');
if (!file.existsSync()) return null;
return file.readAsStringSync();
}