fix: implement debug for meta plugin program and remove unused imports

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

src/meta_plugin/digest.rs

@@ -1,7 +1,6 @@
 use anyhow::Result;
 use sha2::{Digest, Sha256};
 use std::io;
-use std::io::Write;
 use std::time::Instant;
 
 use crate::meta_plugin::MetaPlugin;