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/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)