fix: prevent duplicate metadata storage by returning empty strings from finalize

Co-authored-by: aider (openai/andrew/openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

src/modes/save.rs

@@ -213,7 +213,7 @@ fn process_input_stream(
     for meta_plugin in meta_plugins.iter_mut() {
         match meta_plugin.finalize() {
             Ok(value) => {
-                // Only save non-empty values (empty means already saved)
+                // Only save non-empty values (empty means already saved during initialize/update)
                 if !value.is_empty() {
                     if let Err(e) = meta_plugin.save_meta(conn, item_id, value) {
                         eprintln!("Warning: Failed to save meta value: {}", e);