Charter v64: going back to an older release names the rules it takes back

• Going back to an older release names the rules it takes back (#425, found by QA in its
  review of PR #424). /squad-upgrade vN to a release older than the one the project runs read
  that release's CHANGELOG.md, which has no entry above the running version, and so told DEV, QA
  and the CEO that no rule changed. It now reads the installed playbook's CHANGELOG.md before the
  install replaces it, names the rules that revert, with their § numbers, to DEV and QA and in its
  report, and ends with the headlines of the releases it takes back.
• Upgrading from v63: nothing to do. No rule changes, and the command takes effect at once. A
  restart only brings the squad board's version up to date.