feat(cli): S4 follow-up, refusal backoff and tracker boot (row 45, #1527)

Rocko's round 2 candidate, packet agents/rocko/work/s4-follow-up/
(build.patch c8cec070, candidate manifest 5b067a9d, 8/8 OK).

- Definite DM refusals wait the full 30-minute cap, counted from the
  journal's last refusal, so five refusals span about two hours before
  gave-up (lead decision 73). Unknown outcomes keep doubling.
- Broker close sends at host.mjs:144/150/184/188 pass a callback, which
  closes the EPIPE window both reviewers found in round 1.
- README documents manual recovery for an open decision.
- trackers-boot test, X9, X14, and Darkwing's round 1 notes 1-4.
- The append type check stays out; Rocko's reason holds (both reviewers
  agree).

Reviews: Darkwing approve (comment 26884), Filbert approve (26886).
Landing gate on b13fef4c plus the patch: every node suite and every
scripts/test-*.sh green, test-task 98/0.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/bus-service.sh

@@ -68,6 +68,7 @@ install_unit() {
     fi
     cat <<MSG
 next, for one business (one per data root):
+  mkdir -m 0700 -p <dataRoot>/notify/<business>               the notifier refuses a looser directory
   write <dataRoot>/notify/<business>/notify.json, mode 0600:
     {"notifyVersion": 1, "binding": "<discord binding>"}   or "binding": null for no DMs
   systemctl --user enable --now mosaic-bus@<business>      start now and at login