Say that a deploy keeps only an app's volumes when the app has none (closes #248)

An app with no volume mounts now shows, in its Volume Mounts section,
that the files it writes are lost whenever a deploy or rollback replaces
its container and that a restart keeps them. Each deploy of such an app
writes the same sentence into its log, after the webhook payload and
before the clone. The README says in one sentence that a deploy or
rollback starts a new container that keeps only the files in the app's
volume mounts.

Model: opus-5-5

TODO.md

@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-10-01: An app with no volume mounts says on its page, and in the log of
+  each deploy, that a deploy or rollback loses the files it writes and a restart
+  keeps them; the README says a deploy or rollback keeps only the files the app
+  wrote to its volume mounts (#248).
+
 - 2026-09-29: The app page is 50% wider on large screens (84rem instead of
   56rem), its build log and container log boxes are twice as tall, the build log
   sits between the webhook URL and the environment variables, and the container