fix: remove unused digest field and mutable process

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

src/meta_plugin/program.rs

@@ -96,7 +96,7 @@ impl MetaPlugin for MetaPluginProgram {
 
     fn finalize(&mut self) -> Result<()> {
         debug!("META: Finalizing program plugin");
-        if let Some(mut process) = self.process.take() {
+        if let Some(process) = self.process.take() {
             // Close stdin to signal end of input
             drop(self.writer.take());