feat(provider): ship our own task store, vendored under our own authority

Steps 2 and 3 of docs/STORAGE-AND-SYNC.md. Agendula stops depending on a tasks
provider app being installed: it now carries one.

The module

New :provider — the dmfs task provider 1.4.2 (Apache-2.0, DB 23), vendored
in-tree, renamed to authority de.jeanlucmakiola.agendula.tasks and permissions
de.jeanlucmakiola.agendula.permission.*. It coexists with OpenTasks and
tasks.org rather than replacing them; nothing collides with org.dmfs.*, so both
can be installed at once. The contract shape is untouched — same tables, same
columns — because that is what our data layer and every CalDAV engine already
speak. We own the namespace it lives in, not the schema.

Vendored rather than depended on because the permission names are hardcoded in
the upstream AAR's manifest and cannot be renamed in a prebuilt artifact; in-tree
also satisfies F-Droid's from-source rule. provider/PROVENANCE.md records the
upstream commit and every deviation, each marked with an AGENDULA CHANGE comment
at the site so the list and the code cannot drift apart.

The change that matters most is the account cleanup. Upstream holds GET_ACCOUNTS
and deletes any task list whose account it cannot see. We dropped that permission
— we only ever need our own accounts, which are visible without it — but an
account we cannot see is indistinguishable from one that was removed, so left
alone the provider would quietly delete synced lists. Cleanup is now restricted
to account types this package authenticates itself, which is currently none.
ProviderAccountCleanupTest pins that, and answers open question 3: the local path
works with no account present at all.

Also required by targetSdk 36, none of which upstream faced at 29:
FLAG_IMMUTABLE on the notification PendingIntent, an inexact-alarm fallback so a
revoked SCHEDULE_EXACT_ALARM cannot kill the app on a timezone change, and an
explicit android:exported on the receiver.

Storage modes

ProviderResolver gains a StorageMode: LOCAL (our provider) or EXTERNAL (an
installed one). Not a third SYNCED value — synced is LOCAL with an account
attached, which is derived state, and modelling it as a separate store would
imply switching sync on is a migration. It isn't.

When the user has not chosen, the tell is whether we already hold an external
provider's runtime permission. That permission is dangerous-level, so it can only
be there because an earlier version asked and they agreed — the signature of an
existing Posture A user, who must not be dropped onto an empty store. Fresh
installs get local-first.

hasPermission now short-circuits for our own provider: same-uid access bypasses
the check outright, so ProviderStatus.NEEDS_PERMISSION can no longer fire in
Local mode. That was the work item the storage-and-sync doc called for. The
resolver's platform calls moved behind ProviderEnvironment so the decision — the
part that loses people their data if wrong — is unit-tested on the JVM.

Verified: 51 vendored provider tests pass, app tests pass, lintDebug and
assembleDebug clean. ProviderAccountCleanupTest skips on ARM64, where Robolectric
has no SQLite backend, and runs on x86_64 CI. Not yet exercised on a device.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gradle/libs.versions.toml

@@ -28,6 +28,22 @@ androidxTestRules = "1.7.0"
 # Glance: 1.1.1 is the latest stable (1.2.0 is still rc, 1.3.0 alpha).
 glance = "1.1.1"
 
+# --- :provider (vendored dmfs task provider) ---------------------------------
+# Versions the upstream 1.4.2 source was written against. These are its runtime
+# dependencies, not ours — nothing above the data layer touches them, and they
+# only move when we deliberately resync the fork. See provider/PROVENANCE.md.
+dmfsJems = "1.43"
+dmfsRfc5545Datetime = "0.2.4"
+dmfsLibRecur = "0.12.2"
+# The provider's own test suite is JUnit 4 + Robolectric, unlike the app's
+# JUnit 5. Kept as upstream wrote it (rewriting ~13 test classes would forfeit
+# the regression coverage that makes vendoring safe), but on current versions:
+# upstream pins Robolectric 3.5.1, which predates AGP's resource handling.
+robolectric = "4.16"
+junit4 = "4.13.2"
+hamcrest = "3.0"
+mockito = "5.20.0"
+
 [libraries]
 # AndroidX core
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -92,8 +108,19 @@ androidx-glance-material3 = { group = "androidx.glance", name = "glance-material
 # Android tests - GrantPermissionRule
 androidx-test-rules = { group = "androidx.test", name = "rules", version.ref = "androidxTestRules" }
 
+# :provider — vendored dmfs task provider (see provider/PROVENANCE.md)
+dmfs-jems = { group = "org.dmfs", name = "jems", version.ref = "dmfsJems" }
+dmfs-jems-testing = { group = "org.dmfs", name = "jems-testing", version.ref = "dmfsJems" }
+dmfs-rfc5545-datetime = { group = "org.dmfs", name = "rfc5545-datetime", version.ref = "dmfsRfc5545Datetime" }
+dmfs-lib-recur = { group = "org.dmfs", name = "lib-recur", version.ref = "dmfsLibRecur" }
+robolectric = { group = "org.robolectric", name = "robolectric", version.ref = "robolectric" }
+junit4 = { group = "junit", name = "junit", version.ref = "junit4" }
+hamcrest = { group = "org.hamcrest", name = "hamcrest", version.ref = "hamcrest" }
+mockito-core = { group = "org.mockito", name = "mockito-core", version.ref = "mockito" }
+
 [plugins]
 android-application = { id = "com.android.application", version.ref = "agp" }
+android-library = { id = "com.android.library", version.ref = "agp" }
 kotlin-compose = { id = "org.jetbrains.kotlin.plugin.compose", version.ref = "kotlin" }
 ksp = { id = "com.google.devtools.ksp", version.ref = "ksp" }
 hilt = { id = "com.google.dagger.hilt.android", version.ref = "hilt" }