feat: update default is_internal() to return true and clean up overrides

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/meta_plugin/magic.rs

@@ -118,9 +118,6 @@ impl MagicFileMetaPlugin {
 }
 
 impl MetaPlugin for MagicFileMetaPlugin {
-    fn is_internal(&self) -> bool {
-        true
-    }
     
     fn initialize(&mut self, item_id: i64) -> Result<MetaPluginResponse> {
         self.item_id = Some(item_id);