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/program.rs

@@ -40,6 +40,10 @@ impl MetaPlugin for MetaPluginProgram {
     fn is_supported(&self) -> bool {
         self.supported
     }
+    
+    fn is_internal(&self) -> bool {
+        false
+    }
 
     fn create(&self) -> Result<Box<dyn Write>> {
         debug!("META: Writing using {:?}", *self);