feat: the F-Droid repository moves to halkyon.app

The repository is Halkyon's, for all of its apps, so its address is the
brand's: https://halkyon.app/fdroid/repo. learn.halkyon.app stays Halkyon
Learn's own address, for its website later. Moved before anyone but the
maintainer had added it; the address is permanent once people have.

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

tool/release/fdroid/config.yml

@@ -1,9 +1,10 @@
-# The F-Droid repository at https://learn.halkyon.app/fdroid/repo, kept by
+# The F-Droid repository at https://halkyon.app/fdroid/repo, for all of
+# Halkyon's apps, kept by
 # tool/fdroid_repo.sh: this folder is copied into the repository's working
 # folder (outside Git) before every update. Nothing secret is in here: the index
 # key's path and password come from the environment.
-repo_url: https://learn.halkyon.app/fdroid/repo
-archive_url: https://learn.halkyon.app/fdroid/archive
+repo_url: https://halkyon.app/fdroid/repo
+archive_url: https://halkyon.app/fdroid/archive
 repo_name: Halkyon
 repo_description: >-
   Halkyon's apps, signed by their maintainer: the same APKs as on