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/vars/default.yml

@@ -2,21 +2,12 @@
 # Defaults shared by every Site. Each Site's own file is named by the `var_file`
 # in ansible/hosts, and its slug is its inventory name.
 
-# Which Site config inside the image to run, and therefore which vhost it is
-# for. The file lives in the app repo at config/<site>.site.json.
+# Which Site config inside the image to run, and which web address it serves.
 site: '{{ inventory_hostname }}'
 
 image_name: registry.tanshu.com/menu-admin
 image_tag: "{{ tag | default('latest') }}"
 
-# The hostname Caddy serves this Site on. The parallel run uses a temporary
-# name; cutover points the real one at the app.
-vhost: '{{ host }}'
-
-# Cutover switches. Both stay off until the parallel run has been checked.
-retire_nginx_snippet: false
-retire_nginx_container: false
-
 app_uid: 1000
 app_gid: 1000
 
@@ -24,6 +15,6 @@ caddy_container: caddy
 caddyfile_path: /var/lib/caddy/conf/Caddyfile
 
 caddy_snippet: |
-  {{ vhost }} {
+  {{ host }} {
       reverse_proxy {{ docker_container }}:3000
   }