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/meta_plugin.rs

@@ -8,8 +8,10 @@ use enum_map::enum_map;
 use enum_map::{Enum, EnumMap};
 
 pub mod program;
+pub mod digest;
 
 use crate::meta_plugin::program::MetaPluginProgram;
+use crate::meta_plugin::digest::DigestSha256MetaPlugin;
 
 use strum::IntoEnumIterator;