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>

apps/learn/halkyon_learn_flutter/analysis_options.yaml

@@ -0,0 +1,20 @@
+# 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