release: consume standalone zsh artifacts (#30116)

## Why

Once #30114 publishes zsh independently, regular Rust releases should
reuse that protected, versioned artifact set instead of rebuilding
identical zsh binaries for every Codex version. Keeping the zsh release
tag explicit in the workflow also makes future artifact upgrades
deliberate and easy to review.

This PR assumes the first standalone artifact release will be published
as `codex-zsh-v0.1.0` before this change lands.

## What changed

- Added `CODEX_ZSH_RELEASE_TAG` near the top of
`.github/workflows/rust-release.yml`, initially pinned to
`codex-zsh-v0.1.0`.
- Download the standalone release’s generated `codex-zsh` DotSlash
manifest before assembling Linux and macOS Codex packages.
- Added a `--zsh-manifest` package-builder override so release packaging
fetches the matching target archive and verifies the size and SHA-256
digest recorded in that manifest.
- Removed the reusable zsh build job from regular Rust releases.
- Stopped copying zsh archives into each Rust release and stopped
regenerating a zsh DotSlash manifest there.

Windows packaging remains unchanged because the patched zsh resource is
only shipped for supported Unix targets.

## Testing

- Added package-helper coverage that supplies a standalone manifest
override and verifies the extracted zsh bytes.
- Ran the `scripts/codex_package` unit test suite.
- Validated `.github/scripts/build-codex-package-archive.sh` with `bash
-n`.

.github/scripts/build-codex-package-archive.sh

@@ -9,6 +9,7 @@ Usage: build-codex-package-archive.sh \
   --entrypoint-dir <dir> \
   --archive-dir <dir> \
   [--bwrap-bin <path>] \
+  [--zsh-manifest <path>] \
   [--codex-command-runner-bin <path>] \
   [--codex-windows-sandbox-setup-bin <path>] \
   [--target-suffixed-entrypoint]
@@ -48,6 +49,10 @@ while [[ $# -gt 0 ]]; do
       bwrap_bin_provided="true"
       shift 2
       ;;
+    --zsh-manifest)
+      resource_args+=(--zsh-manifest "${2:?--zsh-manifest requires a value}")
+      shift 2
+      ;;
     --codex-command-runner-bin)
       resource_args+=(
         --codex-command-runner-bin