feat: add is_internal trait method to distinguish internal plugins

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

src/meta_plugin/digest.rs

@@ -22,6 +22,10 @@ impl DigestSha256MetaPlugin {
 }
 
 impl MetaPlugin for DigestSha256MetaPlugin {
+    fn is_internal(&self) -> bool {
+        true
+    }
+    
     fn create(&self) -> Result<Box<dyn Write>> {
         // For meta plugins, we don't actually create a writer since we're buffering data internally
         // This method is required by the trait but not used in the same way as digest engines
@@ -72,6 +76,10 @@ impl ReadTimeMetaPlugin {
 }
 
 impl MetaPlugin for ReadTimeMetaPlugin {
+    fn is_internal(&self) -> bool {
+        true
+    }
+    
     fn create(&self) -> Result<Box<dyn Write>> {
         // For meta plugins, we don't actually create a writer since we're buffering data internally
         Ok(Box::new(DummyWriter))
@@ -115,6 +123,10 @@ impl ReadRateMetaPlugin {
 }
 
 impl MetaPlugin for ReadRateMetaPlugin {
+    fn is_internal(&self) -> bool {
+        true
+    }
+    
     fn create(&self) -> Result<Box<dyn Write>> {
         // For meta plugins, we don't actually create a writer since we're buffering data internally
         Ok(Box::new(DummyWriter))