fix(ci): build gitnexus-shared before npm ci in publish workflow

The publish job failed because npm ci triggers the prepare script (tsc)
before gitnexus-shared types are available. Add the same build step
that the setup-gitnexus composite action uses in CI.

.github/workflows/publish.yml

@@ -30,6 +30,10 @@ jobs:
           registry-url: https://registry.npmjs.org
           cache: npm
           cache-dependency-path: gitnexus/package-lock.json
+      - name: Build gitnexus-shared
+        run: npm install && npm run build
+        working-directory: gitnexus-shared
+
       - run: npm ci
         working-directory: gitnexus