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_flutter/analysis_options.yaml

@@ -1,20 +0,0 @@
-# The workspace's lint rules (see ../analysis_options.yaml), minus the platform folders.
-#
-# If this app ever leaves the pub workspace, it has to enable the analyzer plugin itself:
-# include package:leancode_lint/analysis_options.yaml and add the `plugins:` section here.
-include: ../analysis_options.yaml
-
-analyzer:
-  exclude:
-    - build/**
-    - android/**
-    - ios/**
-    - web/**
-    - windows/**
-    - macos/**
-    - linux/**
-    # Generated by gen-l10n (see l10n.yaml).
-    - lib/l10n/app_localizations*.dart
-
-formatter:
-  trailing_commas: preserve