release: mirror stable tags to the nightly channel

A stable tag only refreshes the stable Homebrew cask; the nightly cask keeps pointing at the last nightly build until the 06:00 UTC cron runs — and create-nightly-tag.sh skips when HEAD already has a nightly, so nightly users can sit a version number below a just-shipped stable release indefinitely.

Add a mirror-nightly job that runs after a successful stable release, cuts a nightly tag from the exact stable commit, and pushes it with a cli-scoped GitHub App token so the push re-triggers the workflow on the nightly-tag path and uploads the entire@nightly cask. The nightly binary stays -nightly-versioned, so versioncheck/brew keep those users on the nightly channel.