refactor: integrate digest functionality into meta plugins and remove digest_engine module

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

src/main.rs

@@ -20,7 +20,6 @@ extern crate lazy_static;
 
 pub mod compression_engine;
 pub mod db;
-pub mod digest_engine;
 pub mod plugins;
 pub mod meta_plugin;
 //pub mod item;