appendFeature method
Persist an updated features list back to srik.yaml.
Strategy: parse the YAML, mutate the in-memory model, and rewrite the
file using the same template format we emit on create. We do not try
to preserve user-added comments — srik.yaml is a managed config file.
Implementation
void appendFeature(String featureName) {
if (features.contains(featureName)) return;
features.add(featureName);
save();
}