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

scripts/codex_package/README.md

@@ -4,6 +4,14 @@ This package contains the implementation behind `scripts/build_codex_package.py`
 The top-level script is the stable executable entry point; these modules keep the
 package-building logic split by responsibility.
 
+Run the builder through `just`:
+
+```bash
+just assemble-codex-package --help
+just assemble-codex-package --variant codex-app-server
+just assemble-codex-package --target x86_64-unknown-linux-gnu
+```
+
 The builder creates a canonical Codex package directory:
 
 ```text

scripts/codex_package/ripgrep.py

@@ -3,12 +3,9 @@
 from pathlib import Path
 
 from .dotslash import fetch_dotslash_executable
-from .targets import REPO_ROOT
-from .targets import TargetSpec
-from .targets import resolve_input_path
+from .targets import TargetSpec, resolve_input_path
 
-
-RG_MANIFEST = REPO_ROOT / "scripts" / "codex_package" / "rg"
+RG_MANIFEST = Path(__file__).with_name("rg")
 
 
 def resolve_rg_bin(spec: TargetSpec, rg_bin: Path | None) -> Path:

scripts/codex_package/targets.py

@@ -1,13 +1,18 @@
 """Supported package targets and default binary discovery."""
 
+import os
 import platform
 import stat
 from dataclasses import dataclass
 from pathlib import Path
 
-
-SCRIPT_DIR = Path(__file__).resolve().parents[1]
-REPO_ROOT = SCRIPT_DIR.parent
+_repo_root = os.environ.get("CODEX_REPO_ROOT")
+if _repo_root is None:
+    raise RuntimeError(
+        "CODEX_REPO_ROOT must point to the repository root; "
+        "run `just assemble-codex-package` to set it automatically"
+    )
+REPO_ROOT = Path(_repo_root)
 
 
 @dataclass(frozen=True)

scripts/codex_package/zsh.py

@@ -3,12 +3,9 @@
 from pathlib import Path
 
 from .dotslash import fetch_dotslash_executable
-from .targets import REPO_ROOT
-from .targets import TargetSpec
-from .targets import resolve_input_path
+from .targets import TargetSpec, resolve_input_path
 
-
-ZSH_MANIFEST = REPO_ROOT / "scripts" / "codex_package" / "codex-zsh"
+ZSH_MANIFEST = Path(__file__).with_name("codex-zsh")
 ZSH_RESOURCE_PATH = Path("zsh") / "bin" / "zsh"