refactor: rename core module to services

Co-authored-by: aider (openai/andrew/openrouter/google/gemini-2.5-pro) <aider@aider.chat>

src/modes/save.rs

@@ -3,7 +3,7 @@ use clap::Command;
 use std::io::{Read, Write};
 
 use crate::config;
-use crate::core::item_service::ItemService;
+use crate::services::item_service::ItemService;
 
 fn validate_save_args(cmd: &mut Command, ids: &Vec<i64>) {
     if !ids.is_empty() {