Deploy in one run, with nothing older to preserve

There is no previous system kept serving, so the playbook no longer knows about
one: the retire flags, the temporary web address, the task that removed the old
nginx snippet and the task that stopped the old container are gone. One run
starts the container and points the Site's web address at it, and deploying an
earlier tag is the rollback. The vars lose vhost, retire_nginx_snippet,
retire_nginx_container and legacy_container; the Caddy role keeps only the
snippet upsert.

The deployment readme drops its staged-rollout sections and the glossary of words
that no longer exist, and the plan's sequence and the operating notes describe
the single run instead.

ansible/playbook.yml

@@ -5,9 +5,9 @@
 #   make build-production TAG=<tag>        (from the repo root)
 #   ansible-playbook --inventory=hosts playbook.yml --limit hopsngrains
 #
-# The first run targets a temporary vhost (menu-new...) so the new site can be
-# checked against the live one. Cutover is the same playbook with the real vhost
-# and the two retire flags set; see readme.md.
+# One run deploys the Site: it starts the container and points the Site's web
+# address at it. Nothing older is kept serving, so there is no staged rollout —
+# deploying an earlier tag is the rollback.
 - hosts: menus
   become: true
   vars_files: