loopd: scheduler yields to a held subagent

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

crates/loopd/src/scheduler.rs

@@ -43,6 +43,9 @@ impl Scheduler {
             self.leftovers_closed = true;
         }
         self.heartbeat(ctx, now, &mut work);
+        if ctx.holds_prefix("sub-") {
+            return work;
+        }
 
         let jobs_dir = ctx.cfg.paths.home.join("jobs");
         let runs_path = jobs_dir.join("runs.jsonl");
@@ -184,6 +187,9 @@ impl Scheduler {
         if !due {
             return;
         }
+        if ctx.holds_prefix("sub-") {
+            return;
+        }
 
         let id = match SessionId::new("heartbeat") {
             Ok(id) => id,