## Why
The package builder already fetches `rg` from a checked-in DotSlash
manifest. The zsh packaging work needs the same
fetch/cache/size-check/SHA-256/extract path for another manifest, but
keeping that refactor inside the zsh PR makes the review harder to
follow.
This PR factors the existing `rg`-specific implementation into a
reusable helper with no intended behavior change for `rg` packaging.
## What Changed
- Added `scripts/codex_package/dotslash.py` for checked-in DotSlash
manifest parsing, archive download, cache reuse, size validation,
SHA-256 validation, and member extraction.
- Updated `scripts/codex_package/ripgrep.py` to delegate to the shared
helper.
- Preserved the existing `rg` manifest path, cache key, destination
filename, and executable-bit behavior.
## Testing
- `python3 -m py_compile scripts/codex_package/dotslash.py
scripts/codex_package/ripgrep.py scripts/codex_package/cli.py
scripts/codex_package/layout.py scripts/codex_package/zsh.py`
- `python3 -m unittest discover scripts/codex_package`
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/24129).
* #23768
* #23756
* __->__ #24129
