NixOS keychain support is broken

### Describe the bug

Copilot fails to access the system keychain despite libsecret, GNOME Keyring and Seahorse being installed.

```
$ copilot login
To authenticate, visit https://github.com/login/device and enter code XXXX-XXXX.
Waiting for authorization...
System keychain unavailable. Store token in plaintext config file? (y/N)
```

### Affected version

GitHub Copilot CLI 1.0.32

### Steps to reproduce the behavior

1. Install libsecret, GNOME Keyring, Seahorse, and Copilot CLI

Home Manager:
```
home.packages = with pkgs; [
libsecret
github-copilot-cli
]
```

NixOS:
```
services.gnome.gnome-keyring.enable = true;
programs.seahorse.enable = true;
```
2. `nixos-rebuild switch`, `reboot`
3. Run `copilot login`

### Expected behavior

That Copilot CLI stores the token with a keychain instead of prompting to store it in plaintext.

### Additional context

```
~ nix-info --markdown
- system: `"aarch64-linux"`
- host os: `Linux 6.18.24, NixOS, 26.05 (Yarara), 26.05.20260427.1c3fe55`
- multi-user?: `yes`
- sandbox: `yes`
- version: `nix-env (Nix) 2.34.6`
- channels(root): `"nixos-25.11"`
- nixpkgs: `/nix/store/l61vfkyy0qrnz9bmgx84fa7z3bjzhyp4-source`

~ fastfetch
...
Host: QEMU Virtual Machine (virt-10.0)
Kernel: Linux 6.18.24
Shell: fish 4.6.0
WM: niri 26.04 (Wayland)
Terminal: foot 1.26.1
CPU: Virtualized Apple Silicon (8)
GPU: RedHat Virtio 1.0 GPU

~ command -v secret-tool
/etc/profiles/per-user//bin/secret-tool
```

Start at the `copilot login` entry point and reproduce the issue on NixOS with libsecret, GNOME Keyring, Seahorse, and `secret-tool` installed. Trace why the system keychain is reported unavailable; done means the token is stored in the keychain without the plaintext-config prompt.

由索引模型根据 Issue 内容生成。