Windows Desktop + WSL: branch switch fails with cannot spawn NUL/post-checkout

## Summary

On Codex Desktop for Windows, switching a WSL-backed repository to main fails because Codex tries to spawn NUL/post-checkout inside WSL.

The UI can still show a previously deleted branch even though the repository's actual HEAD is already on main. Selecting main then produces this toast:

Error switching branch: Already on 'main'
error: cannot spawn NUL/post-checkout: No such file or directory

Codex also pollutes the repository root by creating an untracked NUL/ directory containing Git hook files.

Environment

• Codex Desktop: 26.721.4979.0
• Windows 10 Home 22H2, build 19045.6466
• Repository location: WSL 2 / Ubuntu (\\wsl.localhost\Ubuntu\home\...)
• Git inside WSL: 2.43.0

Steps to reproduce

1. Open a Git repository located inside WSL from Codex Desktop on Windows.
2. Work on a feature branch and later merge/delete that branch.
3. Return to the Codex task. The environment selector may still display the deleted feature branch while git symbolic-ref --short HEAD reports main.
4. Select main in the branch selector.

Actual behavior

The branch selector fails with:

Already on 'main'
error: cannot spawn NUL/post-checkout: No such file or directory

The repository contains:

NUL/
??? post-checkout
??? post-commit
??? post-merge
??? pre-push

git config --show-origin --get core.hooksPath returns no configured value, so these files appear to be created/injected by Codex rather than repository configuration.

Expected behavior

• Codex should refresh the branch selector from the repository's real HEAD.
• Selecting the already-active branch should be a no-op, not an error.
• Hook suppression/path handling must be platform-aware: NUL is a Windows device name, but inside WSL it is a relative path. Codex should use /dev/null or a valid platform-neutral hooks directory when invoking WSL Git.
• Codex should not create an untracked NUL/ directory in the repository.

Suspected cause

A Windows-specific null-device value (NUL) is being passed as a Git hooks path while the Git process runs inside WSL. WSL Git interprets it as <repo>/NUL, which leaves hook files in the working tree and later fails to execute NUL/post-checkout.