feat: Add is_internal method to MetaPlugin trait and implementations

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

src/meta_plugin.rs

@@ -41,6 +41,10 @@ pub trait MetaPlugin {
         true
     }
     
+    fn is_internal(&self) -> bool {
+        false
+    }
+    
     fn create(&self) -> Result<Box<dyn Write>>;
     fn finalize(&mut self) -> io::Result<String>;