chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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

halkyon_learn_client/doc/endpoint.md

@@ -1,15 +0,0 @@
-# Callable endpoints
-
-Each class contains callable methods that will call a method on the server side. These are normally defined in the `endpoint` directory in your server project. This client sends requests to these endpoints and returns the result.
-
-Example usage:
-
-```dart
-// How to use GreetingEndpoint.
-client.greeting.hello("world!");
-
-// Generic format.
-client.<endpoint>.<method>(...);
-```
-
-Please see the full official documentation [here](https://docs.serverpod.dev)