loopd: subagent spawn on the background slot

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/loopd/tests/support/turn.rs

@@ -54,6 +54,7 @@ impl Setup {
             client: &self.client,
             port: &self.port,
             registry: &self.registry,
+            ctx: None,
         }
     }