release: use DotSlash zstd for package archives

[bolinfest]

Why

The Windows release job installed DotSlash successfully, but package archive creation still failed while writing codex-package-*.tar.zst. The Python archiver used shutil.which("zstd"), which does not reliably find the extensionless DotSlash manifest at .github/workflows/zstd from native Windows Python.

That left release packaging dependent on a command named exactly zstd being discoverable on PATH, even though the repository already carries a DotSlash wrapper for Windows runners.

What changed

• Add resolve_zstd_command() to prefer a real zstd binary when present.
• Fall back to invoking dotslash .github/workflows/zstd when zstd is not on PATH.
• Keep the error explicit when neither zstd nor the DotSlash fallback is available.
• Add unit coverage for direct zstd, DotSlash fallback, and missing-tool error paths.

Verification

• python3 -m unittest discover -s scripts/codex_package -p 'test_*.py'
• python3 -m py_compile scripts/codex_package/*.py