docs: architecture tour, docs index, showcase README; ci: Gitea release per tag

Documentation pass after the 2.0 milestone:
- docs/ARCHITECTURE.md — principles (provider as single source of truth,
  observer-driven UI, JVM-first tests, no network), layer + reminder
  mermaid diagrams, navigation (overlay/held-key, no nav lib), and the
  provider lessons (recurring-write invariants, conflict snapshots)
- docs/README.md — map of what documentation lives where, incl. the
  convention that superpowers/ plans are historical artifacts while
  .planning/ stays current
- README.md — showcase layout (centered header, badges, screenshot
  gallery from the fastlane assets, grouped features, install/build/
  architecture/roadmap sections); renders on Gitea
- .planning/{PROJECT,REQUIREMENTS,STATE}.md unstaled: read-only-V1 talk
  removed, V1/V2 checklists marked shipped, state points at v3 + the
  Locations & People go/no-go

release.yaml gains a gitea-release job: on every tag push it extracts the
tag's CHANGELOG section and creates a Gitea release with it as the notes.
No APK assets — distribution stays with the F-Droid repo. Idempotent
(skips an existing release), gated on the test job only so notes appear
even when the F-Droid upload hiccups.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/README.md

@@ -0,0 +1,20 @@
+# Documentation map
+
+Where to look for what:
+
+| Document | What it is |
+|---|---|
+| [`ARCHITECTURE.md`](ARCHITECTURE.md) | Orientation tour: principles, layers, navigation, recurring-write / conflict / reminder pipelines, testing |
+| [`../CHANGELOG.md`](../CHANGELOG.md) | Release history (Keep a Changelog, SemVer) |
+| [`../.planning/ROADMAP.md`](../.planning/ROADMAP.md) | Living roadmap: shipped milestones, current scope, idea backlog |
+| [`../.planning/PROJECT.md`](../.planning/PROJECT.md) | What the project is, stack, naming, infrastructure |
+| [`../.planning/REQUIREMENTS.md`](../.planning/REQUIREMENTS.md) | Requirement checklist per milestone |
+| [`../.planning/STATE.md`](../.planning/STATE.md) | Snapshot of where development currently stands |
+| [`superpowers/specs/`](superpowers/specs/) | The original design spec (2026-06-08) — historical record, not updated |
+| [`superpowers/plans/`](superpowers/plans/) | Per-milestone implementation plans with task checklists — historical record of how each slice was built, including provider lessons learned |
+| [`../fdroid-metadata/`](../fdroid-metadata/) | F-Droid/fastlane store metadata: descriptions, icon, screenshots (DE + EN) |
+
+Conventions: plans and specs under `superpowers/` are point-in-time
+artifacts of the agentic workflow that built each milestone — they get
+status updates but are never rewritten. The `.planning/` files are living
+documents and should stay current.