4.0.0-beta.2

ci: derive npm dist-tag from prerelease label

`npm publish` fails on a prerelease version unless `--tag` is given,
otherwise it would silently land on `latest`. Parse the prerelease
label out of the release tag (4.0.0-beta.1 -> beta, 4.0.0-rc.1 -> rc)
and pass it through; stable releases keep the default `latest`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>