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/keep_pid.rs

@@ -105,7 +105,7 @@ impl MetaPlugin for KeepPidMetaPlugin {
         // Use process_metadata_outputs to handle output mapping
         if let Some(meta_data) = crate::meta_plugin::process_metadata_outputs(
             "keep_pid", 
-            pid, 
+            serde_yaml::Value::String(pid), 
             &self.outputs
         ) {
             metadata.push(meta_data);