[Bug] GPU access blocked in Linux sandbox on WSL2: /dev/dxg not exposed by bubblewrap

Title: [Bug] GPU access blocked in Linux sandbox on WSL2: /dev/dxg not exposed by bubblewrap

Body:

## Summary

When running shell commands inside the Codex Linux sandbox (bubblewrap) on WSL2,
any process that uses NVML (e.g., nvidia-smi, PyTorch CUDA, etc.) fails with:

Failed to initialize NVML: GPU access blocked by the operating system

## Environment

• Codex version: 0.130.0
• OS: WSL2 (kernel 6.6.114.1-microsoft-standard-WSL2)
• GPU: NVIDIA GeForce RTX 4060
• NVIDIA driver (Windows host): 591.86
• WSL nvidia-smi version: 590.57

## Root Cause

Codex launches bubblewrap with --dev /dev, which replaces the container's /dev
with a minimal set:

/dev/null, /dev/zero, /dev/full, /dev/random, /dev/urandom, /dev/tty

On WSL2, GPU access goes through /dev/dxg (the DirectX Graphics kernel interface)
— there are no /dev/nvidia* device nodes. Even though the host filesystem is
bound via --ro-bind / / first, the subsequent --dev /dev overwrites /dev and
hides /dev/dxg, making it inaccessible inside the sandbox.

Confirmed via strace:

execve("/usr/bin/bwrap", ["bwrap", "--new-session", "--die-with-parent",
"--ro-bind", "/", "/",
"--dev", "/dev", ← hides /dev/dxg
"--unshare-user", "--unshare-pid", "--unshare-net",
"--proc", "/proc",
"--", "codex-linux-sandbox", ...], ...)

## Reproduction

```bash
codex sandbox linux -- nvidia-smi
# → Failed to initialize NVML: GPU access blocked by the operating system

Workaround

Setting sandbox_mode = "danger-full-access" in ~/.codex/config.toml bypasses
the user namespace / --dev restriction and allows GPU access.

sandbox_mode = "danger-full-access"

However, this significantly weakens sandboxing.

Proposed Fix

When Codex detects WSL2 (e.g., by checking /proc/version for microsoft or
WSL2, or by detecting /dev/dxg), add a --dev-bind after --dev /dev in the
bubblewrap command to expose the device:

// in linux-sandbox/src/bwrap.rs (approximate)
if Path::new("/dev/dxg").exists() {
bwrap_args.push("--dev-bind");
bwrap_args.push("/dev/dxg");
bwrap_args.push("/dev/dxg");
}

This would restore GPU access inside the sandbox without requiring danger-full-access.