feat: add early binary detection and prevent duplicate metadata saving

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

src/meta_plugin.rs

@@ -74,6 +74,7 @@ pub trait MetaPlugin {
             value,
         };
         crate::db::store_meta(conn, meta)?;
+        self.saved_during_io = true;
         Ok(())
     }
 }