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.sh

@@ -2,7 +2,7 @@
 # Builds a release and publishes it: the Android APK (one for every phone, signed with
 # the app's key) and the Linux snap, both connecting to learn.halkyon.eu, the project's
 # instance; a list of their checksums signed with GPG; all of it as a release on
-# code.halkyon.app, the APK in the F-Droid repository at learn.halkyon.app
+# code.halkyon.app, the APK in the F-Droid repository at halkyon.app
 # (tool/fdroid_repo.sh), and the snap in the Snap Store's edge channel. Run on a
 # developer's laptop, never in CI: the app's key never leaves it.
 #
@@ -198,7 +198,7 @@ the project's instance.
 - **Android:** \`$apk\`, one APK for every phone, signed with the app's key; the \
 SHA-256 of its certificate is \`$certificate\`.
 - **F-Droid:** the same APK in the repository \
-https://learn.halkyon.app/fdroid/repo?fingerprint=$fdroid_fingerprint
+https://halkyon.app/fdroid/repo?fingerprint=$fdroid_fingerprint
 - **Linux:** \`$snap\`, also in the Snap Store: \`snap install halkyon-learn --edge\`.
 
 \`SHA256SUMS\` lists the files' checksums, signed with the GPG key \`$signer\` \