Require only VERCEL_TOKEN; auto-create project on…

Details

Lower the setup friction to a single secret. The deploy step now only links to
an existing project when both VERCEL_ORG_ID and VERCEL_PROJECT_ID are provided;
otherwise `vercel deploy --prebuilt --yes` creates/resolves a project straight
from the token. So the one required secret is VERCEL_TOKEN.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016HyukoK8KXyZw4hY9Wm3zN

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