Add a just recipe for assembling Codex packages (#39584)

## What changed

- Add `just assemble-codex-package` as the documented entry point for the package builder.
- Export `CODEX_REPO_ROOT` from `just` and release CI, and require the package builder to use it for repository-relative inputs.
- Resolve the ripgrep and zsh manifests relative to their Python modules.

GitOrigin-RevId: 039d12107a49171f064b0119adc6b99cf97c6c1e

.github/actions/setup-ci/action.yml

@@ -53,6 +53,7 @@ runs:
         echo "cargo-target-dir=$cargo_target_dir" >> "$GITHUB_OUTPUT"
 
         {
+          echo "CODEX_REPO_ROOT=$GITHUB_WORKSPACE"
           echo "BAZEL_OUTPUT_BASE=$bazel_output_base"
           echo "BAZEL_OUTPUT_USER_ROOT=$bazel_output_user_root"
           echo "BAZEL_REPOSITORY_CACHE=$bazel_repository_cache"