fix: wrap string values in Value::String for process_metadata_outputs

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/meta_plugin/shell.rs

@@ -108,7 +108,7 @@ impl MetaPlugin for ShellMetaPlugin {
         // Use process_metadata_outputs to handle output mapping
         if let Some(meta_data) = crate::meta_plugin::process_metadata_outputs(
             "shell", 
-            shell, 
+            serde_yaml::Value::String(shell), 
             &self.outputs
         ) {
             metadata.push(meta_data);