docs: add architecture, roadmap, releasing, contributing + docs index

Add a grounded doc set under docs/ plus a root CONTRIBUTING.md:
- docs/ARCHITECTURE.md: current code shape (layers, data seam, provider
  resolution, reminder engine, DI, build/tooling, manifest)
- docs/ROADMAP.md: status view (M0/M1 done, M2 in progress, open decisions)
- docs/RELEASING.md: tag-driven release flow, CI jobs, F-Droid repo, secrets
  (was referenced by build.gradle.kts and CHANGELOG but missing)
- docs/README.md: docs index and how the docs relate
- CONTRIBUTING.md: build/test/lint, layer rules, style, PR conventions

Also refresh the stale "M0 — skeleton" status note in README.

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

docs/README.md

@@ -0,0 +1,27 @@
+# Floret — documentation
+
+Floret 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
+[Calendula](https://gitea.jeanlucmakiola.de/makiolaj/calendula). See the
+top-level [`../README.md`](../README.md) for the project pitch.
+
+## Index
+
+| Doc | What it covers |
+|---|---|
+| [`ARCHITECTURE.md`](ARCHITECTURE.md) | How Floret 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. |
+| [`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. |
+
+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").
+- **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).
+- **RELEASING** is the operational runbook.