Add build provenance attestation step to npm publish workflow

🤖 Generated with Claude Code
Co-Authored-By: Claude <[e-mail]>

.github/workflows/npm-publish.yml

···
   release:
     types: [created]
 
+permissions:
+  id-token: write
+  attestations: write
+
 jobs:
   publish:
     runs-on: ubuntu-latest
···
 
       - name: Build
         run: npm run build
+
+      - name: Generate build provenance
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-path: './index.js'
 
       - name: Publish to npm
         run: npm publish --access public