ci: adopt the modern calendula pipeline + Codeberg mirror

Port Calendula's current CI/release pipeline:

- ci.yaml: pull_request-triggered, change-scope classification
  (docs/metadata-only PRs skip the Android build but still report a
  green CI), and a reproducible-release invariant guard.
- release.yaml: the committed versionName is the source of truth — a
  bump reaching main triggers the release, which builds, signs,
  publishes to the F-Droid repo, then mints the vX.Y.Z tag + Gitea
  release and mirrors it to Codeberg with the signed APK + SHA-256
  checksum. workflow_dispatch runs the re-sign-only recovery path.
- Gitea releases are flagged as pre-releases while MAJOR is 0.
- build.gradle.kts: reproducible-release invariants (vcsInfo,
  dependenciesInfo) + a releaseTest variant for the on-device gate.
- fastlane/ becomes the single source of truth for store metadata;
  the localized F-Droid layout is generated from it at release time.
- Port scripts/, .gitea/ISSUE_TEMPLATE/, and rewrite docs/RELEASING.md
  for the versionName-in-main model; fix stale references elsewhere.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fastlane/metadata/android/en-US/full_description.txt

@@ -0,0 +1,10 @@
+Agendula is a modern, open-source task app for Android. It works directly on an
+existing tasks provider (OpenTasks / tasks.org), so any CalDAV tasks synced to
+your device via DAVx5, SmoothSync or DecSync show up automatically, and changes
+you make sync back the same way — no own account, no own sync.
+
+The differentiator is the design: real Material 3 Expressive throughout, with
+dynamic color, expressive motion, and expressive shapes. Sibling to Calendula.
+
+Privacy: zero telemetry, no analytics, no network access of its own — your data
+never leaves the device except through the sync app you already trust.