build(deploy)!: the instance's compose file is compose.learn.yaml, beside compose.train.yaml and compose.todo.yaml

No file has Compose's default name any more, so .env's COMPOSE_FILE is always set,
naming compose.learn.yaml first (.env.example sets it). compose.yaml stays as a link to
compose.learn.yaml for the one deploy that switches learn.halkyon.eu's .env over: the
deploy's merge runs with the COMPOSE_FILE from before. A later commit removes it.

BREAKING CHANGE: an instance's deploy/.env must set COMPOSE_FILE, starting with
compose.learn.yaml, before the link goes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_server/README.md

@@ -26,12 +26,12 @@ EDRDG's licence asks a server showing its dictionaries to update them at least
 once a month:
 
 - **An instance** keeps them current by itself: the `dictionaries` service in
-  `deploy/compose.yaml` (the curator, in a container of its own) downloads
+  `deploy/compose.learn.yaml` (the curator, in a container of its own) downloads
   EDRDG's files of the day once its files are a week old, converts them, and
   replaces them in a volume the server reads (`curate update`, the curator's
   README); the server takes them up within the hour. To supply files yourself
   instead, remove the service and mount `deploy/dictionaries/` into the server
-  (`compose.yaml` says how), and update them monthly.
+  (`compose.learn.yaml` says how), and update them monthly.
 - **Development:** `dart run halkyon_learn_curator:curate update --into
   ../halkyon_learn_server/dictionaries` in the curator's folder (about a
   minute); the server takes them up within the hour.