ci: gate releases on opencode payload verification

.github/workflows/release.yml

@@ -18,6 +18,17 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Setup Node.js
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+        with:
+          node-version: '20.x'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Verify OpenCode package payload
+        run: node tests/scripts/build-opencode.test.js
+
       - name: Validate version tag
         run: |
           if ! [[ "${REF_NAME}" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then