ci: add GitHub Action to auto-publish to npm on release (#8)

* Add GitHub Action to auto-publish to npm on release

Triggered on every GitHub release. Syncs the version from the release
tag (strips the leading 'v') into package.json before publishing, so
maintainers only need to create a release — no manual npm publish step.

Requires an NPM_TOKEN secret with an npm Automation token.

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

* Update actions to latest and use Node 22

- actions/checkout v4 → v6
- actions/setup-node v4 → v6
- Node 20 → 22 (current LTS)

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

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