cm-db: threads — drop unnecessary .iter().copied() on participants slice

Inherited from the a2a merge. Iterating `&[AgentId]` directly yields
`&AgentId`; `AgentId::as_uuid(&self)` works fine on that, so the extra
`.iter().copied()` was pure noise and tripped clippy's
`unnecessary_to_owned` under `-D warnings`.

crates/cm-db/src/repo/threads.rs

@@ -112,7 +112,7 @@ pub async fn create_room(
     .execute(&mut *tx)
     .await?;
     let mut seen = std::collections::HashSet::new();
-    for agent in participants.iter().copied() {
+    for agent in participants {
         if !seen.insert(agent.as_uuid()) {
             continue;
         }