[Fix] Saikuro output directory no longer dependent on generator name

- Ensure necessary directories are available when metrics are configured with `MetricFu::Io::FileSystem.set_directories` called from `configure_metrics`
- Add `MetricFu::Io::FileSystem.scratch_directory(metric)` helper
- Set generator metric_directory to `run_options[:output_directory] || MetricFu::Io::FileSystem.scratch_directory(metric)` (avoids inferring directory names from metric name inferred from generator class name)