docs: M5d-2 given tests: unused import and leftover-held

scheduler_heartbeat imported scheduler::{self} without using it, which
clippy -D warnings would fail at task 06. The live-previous cron tick now
asserts a held leftover stays started.

Co-Authored-By: Grok 4.6 <noreply@x.ai>

docs/plans/M5d2/files/crates/loopd/tests/scheduler_cron.rs

@@ -110,6 +110,12 @@ fn a_live_previous_run_skips_the_tick() {
         }]
     );
     assert!(server.requests_to(CHAT).is_empty());
+    let text = home.read("jobs/runs.jsonl");
+    assert!(text.contains("started"), "{text}");
+    assert!(
+        !text.contains("failed"),
+        "a leftover still held is not closed as failed: {text}"
+    );
     drop(held);
 }