ci: copy root README into nipplejs package before publish

npm uses the README from the package directory for the npmjs.com page.
Copy it from the repo root right before publish so it's included.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yoannmoinet

.github/workflows/release.yaml

@@ -118,6 +118,9 @@ jobs:
 
             - run: yarn test:e2e
 
+            - name: Copy README into package
+              run: cp README.md packages/nipplejs/README.md
+
             - name: Publish
               env:
                   CHANNEL: ${{ steps.channel.outputs.channel }}