feat(release): signed SHA256SUMS.txt for the Linux downloads (#36)

The .deb and .pkg.tar.zst are not signed packages, and the in-app
update prompt has users install them by hand, so nothing checked them.
A new checksums job hashes the Linux downloads into SHA256SUMS.txt,
signs it with the updater's minisign key (tauri signer) and uploads
SHA256SUMS.txt + SHA256SUMS.txt.minisig. The README shows the
verify-then-install command.

Refs #36

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

.gitea/workflows/release.yml

@@ -351,6 +351,52 @@ jobs:
             -H "Content-Type: application/octet-stream" \
             --data-binary @"$PKG"
 
+  # [cinny-desktop #36] The .deb / .pkg.tar.zst aren't signed packages, and
+  # the in-app update prompt has users install them by hand. Publish a
+  # SHA256SUMS.txt for the Linux downloads, signed with the updater's minisign
+  # key, so the install command can verify before installing.
+  checksums:
+    needs: [prepare, build-linux, build-arch]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Hash, sign and upload SHA256SUMS.txt
+        env:
+          TOKEN: ${{ secrets.RELEASE_TOKEN }}
+          RELEASE_ID: ${{ needs.prepare.outputs.release_id }}
+          TAURI_PRIVATE_KEY: ${{ secrets.TAURI_SIGNING_PRIVATE_KEY }}
+          TAURI_PRIVATE_KEY_PASSWORD: ''
+        run: |
+          set -euo pipefail
+          BASE="$GITEA_URL/LotusGuild/cinny-desktop/releases/download/latest"
+          mkdir -p sums && cd sums
+          for f in LotusChat-x86_64.deb LotusChat-x86_64.pkg.tar.zst LotusChat-x86_64.AppImage; do
+            curl -sfL -o "$f" "$BASE/$f"
+          done
+          sha256sum LotusChat-x86_64.deb LotusChat-x86_64.pkg.tar.zst LotusChat-x86_64.AppImage > SHA256SUMS.txt
+          cat SHA256SUMS.txt
+          # The Tauri signer writes <file>.sig (base64 of a minisign signature)
+          # and needs a file extension. Publish the decoded .minisig so users
+          # can run `minisign -Vm` directly.
+          npx --yes @tauri-apps/cli@2.7.1 signer sign SHA256SUMS.txt
+          base64 -d SHA256SUMS.txt.sig > SHA256SUMS.txt.minisig
+          for NAME in SHA256SUMS.txt SHA256SUMS.txt.minisig; do
+            OLD=$(curl -sf "$GITEA_URL/api/v1/repos/$REPO/releases/$RELEASE_ID/assets" \
+              -H "Authorization: token $TOKEN" \
+              | python3 -c "import sys,json; print(next((str(a['id']) for a in json.load(sys.stdin) if a['name']=='$NAME'), ''))" 2>/dev/null || true)
+            [ -n "$OLD" ] && curl -sf -X DELETE \
+              "$GITEA_URL/api/v1/repos/$REPO/releases/$RELEASE_ID/assets/$OLD" \
+              -H "Authorization: token $TOKEN" || true
+            curl -sf -X POST \
+              "$GITEA_URL/api/v1/repos/$REPO/releases/$RELEASE_ID/assets?name=$NAME" \
+              -H "Authorization: token $TOKEN" \
+              -H "Content-Type: application/octet-stream" \
+              --data-binary @"$NAME"
+          done
+
   update-manifest:
     needs: [prepare, build-windows, build-linux]
     runs-on: ubuntu-latest