fix(vercel): keep deployment token out of argv

[WNZhao]

Summary

• authenticate Vercel CLI through its supported VERCEL_TOKEN environment variable
• stop putting the deployment token in process arguments
• add a regression test asserting the secret is absent from argv

Vercel documents VERCEL_TOKEN as the CI authentication path, and argv tokens can be exposed through process inspection or CLI error/suggestion output.

Verification

• pnpm exec vitest run packages/targets/deploy-vercel/src/index.test.ts (8/8)
• pnpm --filter @profullstack/sh1pt-target-deploy-vercel typecheck
• git diff --check