fix(ci): accept pnpm argument separator

[pallyoung]

What changed

• Allow the CLI package validator to ignore pnpm's standalone argument separator.
• Export the argument parser and add a regression test for the exact workflow invocation.

Why

The first 0.5.8 candidate run stopped before npm upload because pnpm forwarded the standalone separator, which the validator treated as an unknown option.

This is release-infrastructure-only and does not change published package contents or require another Changeset/version bump.

Verification

• pnpm exec vitest run --config scripts/vitest.config.ts --environment node scripts/validate-cli-package.test.ts
• git diff --check
• pre-commit Biome checks