[codex] Copy user Bazel settings into Codex worktrees (#25925)

## Why

Codex-created linked worktrees do not include ignored files from the
main worktree. Bazel users who keep local overrides in `user.bazelrc`
therefore lose those settings in every new worktree.

The setup must also work on Windows and must not overwrite a file that
already exists in the worktree.

## What changed

The checked-in Codex environment now invokes
`.codex/environments/setup.py`. The script resolves the main worktree
and current worktree, then uses
`copy_from_main_worktree_to_worktree(repo_relative_path)` to copy
ignored files into new worktrees without overwriting existing
destinations.

`main()` currently copies `user.bazelrc`. Additional repository-relative
paths can be added as further calls to the same helper.

## Validation

- Ran the setup script in a linked worktree and confirmed it handles a
missing main-worktree `user.bazelrc`.
- Verified the helper copies a main-worktree file, preserves an existing
worktree file, and creates parent directories for a nested path.

.codex/environments/environment.toml

@@ -3,7 +3,7 @@ version = 1
 name = "codex"
 
 [setup]
-script = ""
+script = "python ./.codex/environments/setup.py"
 
 [[actions]]
 name = "Run"

.codex/environments/setup.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+
+"""Set up ignored files that should be shared with Codex worktrees."""
+
+import shutil
+import subprocess
+from functools import cache
+from pathlib import Path
+
+
+@cache
+def worktree_paths() -> tuple[Path, Path]:
+    script_dir = Path(__file__).resolve().parent
+    worktree_root = git_path(script_dir / "../..", "--show-toplevel")
+    common_git_dir = git_path(worktree_root, "--git-common-dir")
+    return worktree_root, common_git_dir.parent
+
+
+def git_path(working_directory: Path, argument: str) -> Path:
+    output = subprocess.check_output(
+        [
+            "git",
+            "-C",
+            str(working_directory),
+            "rev-parse",
+            "--path-format=absolute",
+            argument,
+        ],
+        text=True,
+    )
+    return Path(output.strip())
+
+
+def copy_from_main_worktree_to_worktree(repo_relative_path: str) -> None:
+    relative_path = Path(repo_relative_path)
+    if relative_path.is_absolute() or ".." in relative_path.parts:
+        raise ValueError(f"path must be repository-relative: {repo_relative_path}")
+
+    worktree_root, main_worktree = worktree_paths()
+    source_path = main_worktree / relative_path
+    destination_path = worktree_root / relative_path
+
+    print(f"  source: {source_path}")
+    print(f"  destination: {destination_path}")
+
+    if source_path == destination_path:
+        print("  result: running in the main worktree; nothing to copy")
+    elif destination_path.exists():
+        print("  result: destination already exists; nothing to copy")
+    elif not source_path.is_file():
+        print("  result: source does not exist; nothing to copy")
+    else:
+        destination_path.parent.mkdir(parents=True, exist_ok=True)
+        shutil.copy2(source_path, destination_path)
+        print(f"  result: copied {repo_relative_path}")
+
+
+def main() -> None:
+    print("Codex environment setup:")
+    # See codex-rs/docs/bazel.md for the repository's Bazel workflow.
+    copy_from_main_worktree_to_worktree("user.bazelrc")
+
+
+if __name__ == "__main__":
+    main()