fix: change self reference to mutable in save_meta and remove unused imports

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

src/meta_plugin/digest.rs

@@ -3,7 +3,6 @@ use sha2::{Digest, Sha256};
 use std::io;
 use std::io::Write;
 use std::time::Instant;
-use rusqlite::Connection;
 
 use crate::meta_plugin::MetaPlugin;