feat: an F-Droid repository at learn.halkyon.app

Releases also publish the APK in the project's own F-Droid repository,
https://learn.halkyon.app/fdroid/repo. tool/fdroid_repo.sh adds the APK,
signs the indexes with the repository's own key and uploads them to
ramiel as the user fdroid, whose key can only write into the folder
Caddy serves (rrsync), the APKs first and the indexes last.
tool/release.sh checks that key and its password before building and
publishes after the Forgejo release.

- tool/release/fdroid/ holds the repository's settings (Halkyon, the
  snap's icon) and the app's listing (the snap's text), with the
  anti-features TetheredNet (learn.halkyon.eu only, for now) and
  NonFreeNet (YouTube, the server's SSPL) declared with their reasons,
  in F-Droid's own definitions and with its icons (fdroiddata, AGPL v3).
- The index key's certificate is pinned: a new key would make the F-Droid
  app refuse the repository.
- fdroidserver v2.4.5, since Ubuntu's v2.2.1 knows neither TetheredNet
  nor reasons.

README and AGENTS.md describe it.

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/metadata/app.halkyon.learn.yml

@@ -0,0 +1,25 @@
+# Halkyon Learn's listing in the F-Droid repository: the snap's summary and
+# description (snap/snapcraft.yaml), and its anti-features with their reasons.
+Categories:
+  - Education
+License: AGPL-3.0-or-later
+AuthorName: Halkyon
+AuthorEmail: support@halkyon.app
+WebSite: https://learn.halkyon.app
+SourceCode: https://code.halkyon.app/project-halkyon/halkyon_learn
+
+AntiFeatures:
+  TetheredNet: It connects to learn.halkyon.eu, the project's instance, and can't be
+    pointed at another one yet.
+  NonFreeNet: YouTube videos in study material play through YouTube, and the
+    server's licence (SSPL) isn't a free licence by F-Droid's definition.
+
+Name: Halkyon Learn
+Summary: Learn with flashcards, studied offline and synced between your devices
+Description: |-
+  Halkyon Learn is a spaced-repetition learning app: studysets of flashcards with
+  rich text, notes, learning paths and PDFs, reviewed on an FSRS schedule. It works
+  offline and syncs between your devices.
+
+  It starts with an anonymous account; signing in with GitHub later keeps
+  everything. It connects to learn.halkyon.eu, the project's instance.