loopd: subagent spawn on the background slot

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

crates/loopd/tests/compact_turn.rs

@@ -75,6 +75,7 @@ impl World {
             client: &self.client,
             port: &self.port,
             registry: &self.registry,
+            ctx: None,
         }
     }
 
@@ -244,6 +245,7 @@ fn a_successful_turn_stays_successful_when_compact_fails() {
         client: &client,
         port: &port,
         registry: &registry,
+        ctx: None,
     };
     let mut events = Vec::new();
     let outcome = run_turn(&mut s, &rt, "hi", None, &mut |e| events.push(e.clone()));