add annotation to snapshot branch name

[pcal43]

after the date, just a note about the snapshot. could be provided by hand via commands or automatically

/backup now rebuilding in progress

  snapshot/4c04bd85-c2ab-402c-b6da-0a218d0e6d22/2022-09-08_18-21-06/scheduled-backup
  snapshot/4c04bd85-c2ab-402c-b6da-0a218d0e6d22/2022-09-08_18-18-31/finished-castle
  snapshot/4c04bd85-c2ab-402c-b6da-0a218d0e6d22/2022-09-08_18-20-20/creeper-destruction
  snapshot/4c04bd85-c2ab-402c-b6da-0a218d0e6d22/2022-09-08_18-21-06/shutdown
  snapshot/4c04bd85-c2ab-402c-b6da-0a218d0e6d22/2022-09-08_18-24-14/rebuilding-in-progress

[pcal43]

or maybe just any time you can do

/backup note rebuilding in progress

gets picked up at next backup (including scheduled/shutdown)