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/android/build.gradle.kts

@@ -1,24 +0,0 @@
-allprojects {
-    repositories {
-        google()
-        mavenCentral()
-    }
-}
-
-val newBuildDir: Directory =
-    rootProject.layout.buildDirectory
-        .dir("../../build")
-        .get()
-rootProject.layout.buildDirectory.value(newBuildDir)
-
-subprojects {
-    val newSubprojectBuildDir: Directory = newBuildDir.dir(project.name)
-    project.layout.buildDirectory.value(newSubprojectBuildDir)
-}
-subprojects {
-    project.evaluationDependsOn(":app")
-}
-
-tasks.register<Delete>("clean") {
-    delete(rootProject.layout.buildDirectory)
-}