fix(release): mark RC tags as GitHub prereleases (#268)

softprops/action-gh-release@v2 only auto-detects prereleases when
semver.valid() recognizes the tag. The grok-dev@X.Y.Z-rc* tag format
has a non-semver prefix, so detection silently fails and RCs get
published as full releases. GitHub's /releases/latest endpoint —
which install.sh uses to resolve the default install version — then
serves pre-release builds to curl | bash users by default.

Classify prereleases explicitly via contains() so -rc, -alpha, -beta,
and -pre tags are flagged at publish time. No shell step, no injection
surface — evaluated at workflow-parse time using native expression
functions.

Co-authored-by: Veeti Hietasalo <veeti.hietasalo@mad3d.ai>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>