feat: update finalize to return Result<()> and simplify save mode

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/meta_plugin/magic.rs

@@ -114,15 +114,14 @@ impl MetaPlugin for MagicFileMetaPlugin {
         Ok(())
     }
 
-    fn finalize(&mut self) -> io::Result<String> {
+    fn finalize(&mut self) -> Result<()> {
         // Save all magic metadata if not already saved
         if !self.is_saved {
             if let Err(e) = self.save_all_magic_metadata() {
                 eprintln!("Warning: Failed to save magic metadata: {}", e);
             }
         }
-        // Return empty string since we save during finalize or update
-        Ok("".to_string())
+        Ok(())
     }
 
     fn update(&mut self, data: &[u8]) {