Today's three skills-index PRs (#33748, #33809, #34025) merged to main
but the live Vercel-hosted docs site didn't pick them up — Vercel is
fired by the deploy-vercel job, which was gated on release events only.
Out-of-band main commits between releases couldn't reach Vercel without
cutting a tag.
Widen the gate to also include workflow_dispatch so 'gh workflow run
deploy-site.yml' can ship pending main changes to Vercel on demand.
Release-tag behavior is unchanged.