ci: skip the npm publish when the version is already released

A version already on npm cannot be republished, so re-running the publish job —
after re-pushing a tag, or a manual re-run — failed on a duplicate version.
Check the registry first and skip instead, so pushing a tag is always safe and
the tag stays the source of truth.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>