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

justfile

@@ -1,5 +1,6 @@
 set working-directory := "codex-rs"
 set positional-arguments
+export CODEX_REPO_ROOT := justfile_directory()
 export JUST_SHELL := justfile_directory() / "scripts/just-shell.py"
 set shell := ["python3", "-c", 'import os, runpy; runpy.run_path(os.environ["JUST_SHELL"], run_name="__main__")']
 set windows-shell := ["python", "-c", 'import os, runpy; runpy.run_path(os.environ["JUST_SHELL"], run_name="__main__")']
@@ -35,6 +36,11 @@ file-search *args:
 code-mode-host *args:
     cargo run --bin codex-code-mode-host -- {args}
 
+# Assemble a local Codex package.
+[no-cd]
+assemble-codex-package *args:
+    {{ python }} {{ justfile_directory() }}/scripts/build_codex_package.py {args}
+
 # Build the CLI and run the app-server test client
 app-server-test-client *args:
     cargo build -p codex-cli