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/config/schema.rs

@@ -1810,7 +1810,6 @@ fn sync_directory(_path: &Path) -> Result<()> {
 mod tests {
     use super::*;
     use std::path::PathBuf;
-    use tempfile::TempDir;
 
     // ── Defaults ─────────────────────────────────────────────