chore(lint): remove unused imports, variables, and redundant mut bindings

Eliminate low-risk clippy warnings as part of the strict lint backlog (#409):

- Remove unused `uuid::Uuid` imports from slack and telegram channels
- Remove unnecessary `mut` and redundant rebindings in agent loop
- Prefix unused `channel_id` variable in discord channel
- Remove unused test imports (`ChatResponse`, `ToolCall`, `TempDir`, `Path`)

src/channels/telegram.rs

@@ -3,7 +3,6 @@ use async_trait::async_trait;
 use reqwest::multipart::{Form, Part};
 use std::path::Path;
 use std::time::Duration;
-use uuid::Uuid;
 
 /// Telegram's maximum message length for text messages
 const TELEGRAM_MAX_MESSAGE_LENGTH: usize = 4096;