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/tools/git_operations.rs

@@ -2,8 +2,6 @@ use super::traits::{Tool, ToolResult};
 use crate::security::{AutonomyLevel, SecurityPolicy};
 use async_trait::async_trait;
 use serde_json::json;
-#[cfg(test)]
-use std::path::Path;
 use std::sync::Arc;
 
 /// Git operations tool for structured repository management.