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/roles/caddy/tasks/main.yaml

@@ -7,33 +7,16 @@
 - name: Fail when Caddy is not where we expect it
   fail:
     msg: >-
-      {{ caddyfile_path }} is missing. Deploy the caddy playbook (which owns the
-      Caddy container and its config) before this one.
+      {{ caddyfile_path }} is missing. Deploy the caddy playbook, which owns the
+      Caddy container and its config, before this one.
   when: not menu_caddyfile.stat.exists
 
-# blockinfile replaces whatever sits between its markers, so changing `vhost`
-# updates the snippet in place. The sibling nginx-menu role only appended a
-# block when its exact text was absent, which cannot change a proxy target.
-- name: Point Caddy at the app
+# blockinfile replaces whatever sits between its markers, so redeploying updates
+# the snippet in place rather than appending a second one.
+- name: Point the Site's web address at the app
   blockinfile:
     path: '{{ caddyfile_path }}'
     marker: '# {mark} ANSIBLE MANAGED: menu app ({{ site }})'
     block: '{{ caddy_snippet }}'
     create: no
   notify: Reload Caddy
-
-- name: Remove the snippet the nginx-menu playbook wrote for this vhost
-  # A different marker, written by a different role. Leaving it in place would
-  # leave Caddy with two blocks for the same hostname.
-  blockinfile:
-    path: '{{ caddyfile_path }}'
-    marker: '# {mark} Ansible managed Caddy snippet for {{ host }}'
-    state: absent
-  when: retire_nginx_snippet | bool
-  notify: Reload Caddy
-
-- name: Stop the old nginx container
-  docker_container:
-    name: '{{ legacy_container }}'
-    state: stopped
-  when: retire_nginx_container | bool