always write ts, when a todo with schedule deleted, conditionally go back to schedule for loopers

cmd/pttodo-cli/testdata/test.yaml

@@ -1,13 +1,20 @@
 todo:
-- b
+- todo: b
+  ts: Thu Mar 24 09:03:24 MDT 2022
 scheduled:
+- todo: due
+  schedule: "2022-05-01"
+  ts: Thu Mar 24 09:16:21 MDT 2022
+- todo: loop
+  schedule: 10s
+  ts: Thu Mar 24 09:17:11 MDT 2022
+done:
 - todo: abc
   schedule: "2022-05-01"
   ts: Wed Mar 23 08:03:45 MDT 2022
 - todo: def
   schedule: "2022-05-01"
   ts: Wed Mar 23 08:04:05 MDT 2022
-done:
 - todo: other
   ts: Wed Mar 23 09:44:57 MDT 2022
 - todo: b
@@ -18,3 +25,21 @@ done:
   ts: Wed Mar 23 09:45:24 MDT 2022
 - todo: a
   ts: Wed Mar 23 09:45:31 MDT 2022
+- todo: looper
+  schedule: 15s
+  ts: Thu Mar 24 09:01:41 MDT 2022
+- todo: looper
+  schedule: 15s
+  ts: Thu Mar 24 09:04:51 MDT 2022
+- todo: loop
+  schedule: 10s
+  ts: Thu Mar 24 09:06:59 MDT 2022
+- todo: due
+  schedule: "2022-02-01"
+  ts: Thu Mar 24 09:15:42 MDT 2022
+- todo: due
+  schedule: "2022-02-01"
+  ts: Thu Mar 24 09:16:53 MDT 2022
+- todo: due
+  schedule: "2022-03-01"
+  ts: Thu Mar 24 09:17:07 MDT 2022