refactor: change meta_name to immutable reference and add debugging

Co-authored-by: aider (openai/andrew/openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

src/meta_plugin/magic.rs

@@ -156,7 +156,7 @@ impl MetaPlugin for MagicFileMetaPlugin {
         }
     }
 
-    fn meta_name(&mut self) -> String {
+    fn meta_name(&self) -> String {
         "magic_file".to_string()
     }