fix(ci): retry legacy CLI update checks

[pallyoung]

What changed

• Retry the legacy CLI update start command while the previous version's automatic startup check is still in progress.
• Limit retries to the exact update_busy condition and a bounded 30-second window.
• Add regression coverage for the observed 0.5.6 startup race.

Why

The 0.5.8 candidate acceptance reached the packaged update flow but one attempt failed with update_busy because the legacy 0.5.6 server was still completing its automatic startup update check. The acceptance harness should wait for that transient check instead of treating it as an installation failure.

This changes only the repository release acceptance scripts. It does not change files included in the published CLI package, so the immutable npm 0.5.8 candidate remains reusable and no Changeset is required.

Verification

• pnpm exec vitest run --config scripts/vitest.config.ts --environment node scripts/verify-cli-update.test.ts (11/11)
• pnpm ci:typecheck
• pnpm ci:build
• pnpm exec biome check scripts/verify-cli-update.ts scripts/verify-cli-update.test.ts
• pnpm validate:cli-package against a real packed 0.5.8 tarball

Local full script tests retain existing Windows-only failures around symlink permissions, POSIX paths/executable bits, and the pre-commit subprocess; Linux PR CI is the authoritative full run.