docs: decide to build our own store and delete the vendored provider

The vendored dmfs provider was kept on the grounds that it hands us the
sync bookkeeping for free. The phase-1 sync audit measured that
bookkeeping and found most of it broken, absent, or unusable: _DIRTY not
set on delete, no home for a per-collection sync token, read-only
collections inexpressible, ACCOUNT_TYPE write-once so enabling sync is a
full migration, and cleanUpLists able to delete a user's lists after a
backup restore. Sixteen findings are provider-imposed rather than
platform- or protocol-imposed.

Costing the alternative showed the swap is far smaller than assumed.
TasksDataSource is already a 14-method, domain-shaped interface;
exactly one file above the data layer references TasksContract. The
work is a second implementation behind an interface built for it, not a
rewrite. Against ~5 weeks to build, owning the store removes 2.5-4
weeks from the sync plan, and 8,200 of the vendored 14,555 lines are
things we would never write - 23 migrations from a 2013 schema, 798
lines of full-text search the app has zero call sites for, and 1,581
lines of a type-safe layer over ContentValues that Room deletes.

External mode (OpenTasks, tasks.org) is unaffected and keeps every
file that describes somebody else's schema.

STORAGE-DECISION.md is the reasoning; OWN-STORE.md is the architecture
and the six-phase plan. :provider stays in-tree until phase 5 so
recurrence parity can be tested against it before it goes.

Also corrected here: the provider's JVM test count (51 -> 56, measured
from the test-results XML) and a fourth site of the debunked "switching
sync on is never a migration" claim, in StorageMode.kt.

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

docs/README.md

@@ -1,8 +1,10 @@
 # Agendula — documentation
 
-Agendula is a Material 3 Expressive **task** app for Android: a pure front-end over
-the OpenTasks `TaskContract` provider (synced by DAVx5 / SmoothSync / DecSync
-over CalDAV), with no own database or sync stack. Sibling to
+Agendula is a Material 3 Expressive **task** app for Android. It **carries its
+own task store** — the dmfs task provider vendored under our own authority — so
+it is complete and local-first with nothing else installed; an external provider
+(OpenTasks / tasks.org, synced by DAVx5 / SmoothSync / DecSync) is a user choice
+rather than a requirement, and our own CalDAV sync is the 1.x arc. Sibling to
 [Calendula](https://codeberg.org/jlmakiola/calendula). See the
 top-level [`../README.md`](../README.md) for the project pitch.
 
@@ -12,15 +14,24 @@ top-level [`../README.md`](../README.md) for the project pitch.
 |---|---|
 | [`ARCHITECTURE.md`](ARCHITECTURE.md) | How Agendula is built **today** — layers, the data seam, provider resolution, the reminder engine, DI, build/tooling, manifest. Start here to work on the code. |
 | [`ROADMAP.md`](ROADMAP.md) | **Status** and what's next — milestones (M0–M6 + Posture B), what's done, open decisions, how to build/verify. |
+| [`STORAGE-AND-SYNC.md`](STORAGE-AND-SYNC.md) | **Where task data lives** — the decision to ship our own provider, the storage modes, permissions, distribution, and the dead ends. Supersedes `PLAN.md` on storage. |
+| [`SYNC.md`](SYNC.md) | **How data reaches a server** — the CalDAV sync adapter: the VTODO ↔ `TaskContract` mapper, Nextcloud sign-in, the engine, libraries and their licenses. Step 5 of `STORAGE-AND-SYNC.md`. |
+| [`STORAGE-DECISION.md`](STORAGE-DECISION.md) | **Keep the vendored provider, or build our own?** The measured cost of both. **Decided: build our own.** |
+| [`OWN-STORE.md`](OWN-STORE.md) | **Agendula's own Room store** — the schema, recurrence design, migration off the vendored provider, and the six-phase plan that deletes `:provider`. Supersedes the "keep the provider" position in `STORAGE-AND-SYNC.md`. |
 | [`PLAN.md`](PLAN.md) | The original implementation plan and **design rationale** — the A-now-B-later thesis, what transfers from Calendula, the locked decisions. The "why". |
 | [`RELEASING.md`](RELEASING.md) | How to cut a release — the git-tag-as-source-of-truth flow, CI jobs, F-Droid repo, required secrets. |
+| [`../provider/PROVENANCE.md`](../provider/PROVENANCE.md) | What the vendored `:provider` module is, where it came from, and **every** deviation from upstream dmfs. |
 
 Also: [`../CHANGELOG.md`](../CHANGELOG.md) (Keep a Changelog format; tag sections
 feed the release notes).
 
 ## How the docs relate
 
-- **PLAN** is the design decisions (mostly stable; the "why").
+- **PLAN** is the original design decisions (the "why"), left as the historical
+  record. On storage it is **superseded by STORAGE-AND-SYNC**.
+- **STORAGE-AND-SYNC** and **SYNC** are the standing decision documents: the
+  first settles where data lives, the second how it syncs. Both record rejected
+  alternatives on purpose, so decisions don't get relitigated.
 - **ARCHITECTURE** is the current shape of the code (kept in sync with the
   source as it grows).
 - **ROADMAP** is the moving status layer (update as milestones land).