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>

.gitea/workflows/release.yaml

@@ -1,4 +1,4 @@
-name: Build and Release to F-Droid
+name: Release — F-Droid repo + Gitea release
 
 on:
   push:
@@ -222,3 +222,68 @@ jobs:
           -mkdir dev/fdroid/repo
           SFTP
           sshpass -p "$PASS" scp $SSH_OPTS -r fdroid/. "$USER@$HOST:dev/fdroid/"
+
+  # A Gitea release per tag, carrying the tag's CHANGELOG section as its
+  # notes. Deliberately no APK assets — distribution stays with the F-Droid
+  # repo; the release is the human-readable record. Gated on the tests-only
+  # ci job (not the deploy) so notes appear even if the F-Droid upload has
+  # an infrastructure hiccup.
+  gitea-release:
+    needs: ci
+    if: startsWith(github.ref, 'refs/tags/')
+    runs-on: docker
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Extract changelog section for this tag
+        run: |
+          set -e
+          TAG="${GITHUB_REF_NAME:-${GITHUB_REF##*/}}"
+          VERSION="${TAG#v}"
+          # Everything between "## [<version>]" and the next "## [" heading.
+          awk -v ver="$VERSION" '
+            $0 ~ "^## \\[" ver "\\]" { flag = 1; next }
+            /^## \[/ { flag = 0 }
+            flag' CHANGELOG.md > release-notes.md
+          # Trim leading blank lines.
+          sed -i -e '/./,$!d' release-notes.md
+          if [ ! -s release-notes.md ]; then
+            echo "_No changelog entry for ${VERSION} — see CHANGELOG.md._" > release-notes.md
+          fi
+          echo "--- release notes ---"
+          cat release-notes.md
+
+      - name: Create Gitea release
+        env:
+          TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          API: ${{ github.server_url }}/api/v1/repos/${{ github.repository }}
+        run: |
+          set -e
+          TAG="${GITHUB_REF_NAME:-${GITHUB_REF##*/}}"
+          # Re-runs must not fail on an already-published release.
+          STATUS=$(curl -s -o /dev/null -w '%{http_code}' \
+            -H "Authorization: token $TOKEN" "$API/releases/tags/$TAG")
+          if [ "$STATUS" = "200" ]; then
+            echo "Release for $TAG already exists — skipping."
+            exit 0
+          fi
+          python3 - "$TAG" <<'PY' > payload.json
+          import json, sys
+          print(json.dumps({
+              "tag_name": sys.argv[1],
+              "name": sys.argv[1],
+              "body": open("release-notes.md").read(),
+              "draft": False,
+              "prerelease": False,
+          }))
+          PY
+          CODE=$(curl -s -o response.json -w '%{http_code}' -X POST \
+            -H "Authorization: token $TOKEN" \
+            -H "Content-Type: application/json" \
+            -d @payload.json "$API/releases")
+          cat response.json
+          if [ "$CODE" != "201" ]; then
+            echo "Release creation failed with HTTP $CODE"
+            exit 1
+          fi