## 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`
Michael Bolin