Michael Bolin 9dd45d0217 install: consume Codex package archives ...

## Summary

Standalone installs should exercise the same canonical package archive layout that release builds produce, rather than unpacking npm platform packages and reconstructing a parallel install tree.

This updates `install.sh` and `install.ps1` to download `codex-package-<target>.tar.gz` plus `codex-package_SHA256SUMS`, verify the selected archive against the manifest, install the archive directly, and point visible command shims at `bin/codex` inside the package layout. Existing installs are still version-detected through the old root-level `codex` path as a fallback during migration.

## Test plan

- `sh -n scripts/install/install.sh`
- `pwsh -NoProfile -Command '<PowerShell parser smoke test for scripts/install/install.ps1>'`

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..

install.ps1

install: consume Codex package archives

2026-05-19 23:35:10 -07:00

install.sh

install: consume Codex package archives

2026-05-19 23:35:10 -07:00