M1: full task data layer, reminders and ViewModels over OpenTasks

Non-visual stack (backoffice) complete and verified against the real
provider semantics (tasks.org's bundled dmfs TaskProvider, authority
org.tasks.opentasks, org.tasks.permission.* dangerous):

- TaskContract subset + ProviderResolver (runtime authority/permission)
- AndroidTasksDataSource (Instances query, ContentObserver) + TasksRepository
  with live Flows; domain models, smart-list filtering, sorting, form validation
- Self-scheduled due-reminder engine (AlarmManager, boot/provider-change)
- Render-only ViewModels + UiState for every screen
- 24 JVM unit tests; assembleDebug + lintDebug + testDebugUnitTest green
- RootScreen is a functional scaffold over real data, to be replaced with the
  Material 3 Expressive screens one by one

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,21 @@ the source of truth for version codes (see Calendula's `docs/RELEASING.md`).
 ## [Unreleased]
 
 ### Added
+- M1 data layer + logic ("backoffice"): the full non-visual stack over the
+  OpenTasks `TaskContract` provider (authority resolved at runtime —
+  `org.tasks.opentasks` for tasks.org, `org.dmfs.tasks` for OpenTasks).
+  - Vendored `TaskContract` subset, `ProviderResolver`, `ColumnReader`, mappers,
+    `AndroidTasksDataSource` (Instances query + ContentObserver), and
+    `TasksRepository` exposing live Flows of lists/tasks/detail.
+  - Domain models, smart-list filtering (Today/Upcoming/Overdue/No-date/All/
+    Completed), sorting, form validation, sub-tasks via `parent_id`.
+  - Self-scheduled due-reminder engine (AlarmManager + boot/provider-change
+    re-sync), notifications, DataStore prefs.
+  - Render-only ViewModels + UiState for every screen (lists, task list, detail,
+    edit, settings, permission) so the UI is build-only.
+  - 24 JVM unit tests (mappers, filtering, sorting, form, value mapping, day
+    windows). `RootScreen` is a functional scaffold over the real data, to be
+    replaced screen-by-screen with the Material 3 Expressive design.
 - M0 skeleton: project scaffolding copied from Calendula (Gradle, version
   catalog, Hilt, Material 3 Expressive theme, Gitea CI/release workflows),
   reseeded to a warm-mauve palette. Builds and shows a themed placeholder.