feat: add default_options method to all meta plugins

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

src/meta_plugin/magic.rs

@@ -149,5 +149,11 @@ impl MetaPlugin for MagicFileMetaPlugin {
     fn default_outputs(&self) -> Vec<String> {
         vec!["mime_type".to_string(), "mime_encoding".to_string(), "file_type".to_string()]
     }
+    
+    fn default_options(&self) -> std::collections::HashMap<String, serde_yaml::Value> {
+        let mut options = std::collections::HashMap::new();
+        options.insert("max_buffer_size".to_string(), serde_yaml::Value::Number(4096.into()));
+        options
+    }
 }