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/config/antiFeatures.yml

@@ -0,0 +1,13 @@
+# The anti-features the repository's apps declare, as the main F-Droid
+# repository defines them: names, descriptions and icons from fdroiddata
+# (https://gitlab.com/fdroid/fdroiddata, config/antiFeatures.yml and the icons
+# beside it). The two icons are F-Droid's, under the GNU AGPL v3, fdroiddata's
+# licence. fdroid copies them into the repository's icons.
+NonFreeNet:
+  name: Non-Free Network Services
+  description: This app promotes or depends entirely on a non-free network service
+  icon: ic_antifeature_nonfreenet.png
+TetheredNet:
+  name: Tethered Network Services
+  description: This app depends entirely on a certain instance of a network service
+  icon: ic_antifeature_tetherednet.png

tool/release/fdroid/config/categories.yml

@@ -0,0 +1,3 @@
+# The categories the repository's apps are listed in.
+Education:
+  name: Education