refactor: remove is_default from meta plugins and add read_time/read_rate plugins

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

src/meta_plugin/digest.rs

@@ -40,9 +40,6 @@ impl MetaPlugin for DigestSha256MetaPlugin {
         self.meta_name.clone()
     }
 
-    fn is_default(&self) -> bool {
-        true
-    }
 }
 
 // Dummy writer that implements Write but doesn't do anything