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/slack.rs

@@ -1,6 +1,5 @@
 use super::traits::{Channel, ChannelMessage};
 use async_trait::async_trait;
-use uuid::Uuid;
 
 /// Slack channel — polls conversations.history via Web API
 pub struct SlackChannel {