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/mod.rs

@@ -1065,7 +1065,7 @@ mod tests {
     use super::*;
     use crate::memory::{Memory, MemoryCategory, SqliteMemory};
     use crate::observability::NoopObserver;
-    use crate::providers::{ChatMessage, ChatResponse, Provider, ToolCall};
+    use crate::providers::{ChatMessage, Provider};
     use crate::tools::{Tool, ToolResult};
     use std::collections::HashMap;
     use std::sync::atomic::{AtomicUsize, Ordering};