package: include zsh fork in Codex package

Teach the Codex package builder to fetch the prebuilt zsh fork from a checked-in DotSlash manifest and install it under codex-resources/zsh/bin/zsh when an artifact is available for the package target.

Generalize the DotSlash download/cache/verify helper previously embedded in ripgrep packaging so additional checked-in DotSlash manifests can use the same SHA-256 and size validation path. Add install-context support for locating the bundled zsh fork and thread that path through config loading as the lowest-precedence zsh_path default, preserving explicit CLI/profile/global config values.

Also avoid preserving platform-specific file metadata when copying executables into the package directory so package smoke tests can use macOS system binaries as inputs.

scripts/codex_package/targets.py

@@ -40,6 +40,7 @@ class PackageVariant:
 class PackageInputs:
     entrypoint_bin: Path
     rg_bin: Path
+    zsh_bin: Path | None
     bwrap_bin: Path | None
     codex_command_runner_bin: Path | None
     codex_windows_sandbox_setup_bin: Path | None