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-index-certificate.sha256

@@ -1,5 +1,5 @@
 # The SHA-256 of the certificate that signs the indexes of the F-Droid repository at
-# https://learn.halkyon.app/fdroid/repo (key alias halkyon-fdroid), as keytool prints it.
+# https://halkyon.app/fdroid/repo (key alias halkyon-fdroid), as keytool prints it.
 # The F-Droid app knows the repository by it; tool/fdroid_repo.sh uploads no index
 # signed otherwise.
 E8:08:27:88:75:6F:6B:7E:68:2A:A2:2F:CE:AD:24:B5:3E:2A:8B:09:12:AD:66:D6:5E:8C:2D:4F:07:BB:94:B0

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