update README with new modular file structure and fix unused import

- Updated Project Structure section to reflect new modular architecture
- Added detailed module breakdown (cli, files, gemini, models, settings, storage)
- Included Module Overview with descriptions of each module's purpose
- Removed HACKATHON_REVIEW.md file
- Fixed unused import warning in src/files/batch.rs

src/files/batch.rs

@@ -45,7 +45,6 @@ impl FileBatch {
 mod tests {
     use super::*;
     use std::fs::{self, File};
-    use std::path::Path;
 
     #[test]
     fn test_file_batch_from_path() {