Regression: 'XDG_CONFIG_HOME' ignored -- no equivalent available

### Describe the bug

The `XDG_CONFIG_HOME` environment variable no longer appears to be respected for determining the configuration directory. This is a regression from 0.0.421.

Note that `COPILOT_HOME` and `--config-dir` are not acceptable substitutes because they also redirect machine local state, including:

* The SQLite database
* Auto-update package cache under pkg/
* Temporary .lock files

These artifacts are unsuitable for cloud sync (OneDrive) or version control (Git).

By contrast, XDG_CONFIG_HOME would only redirect the user's configuration (config.json, skills, agents, etc.). This is the motivation for separating XDG_CONFIG_HOME and XDG_STATE_HOME in the XDG spec.

### Affected version

GitHub Copilot CLI 1.0.4

### Steps to reproduce the behavior

1. Set XDG_CONFIG_HOME

```ps1
# PowerShell (Windows) — set for current session
$env:XDG_CONFIG_HOME = "D:\my_copilot_config"
```

2. Create a minimal agent under the XDG config path

```ps1
# PowerShell
New-Item -ItemType Directory -Force "$env:XDG_CONFIG_HOME\.copilot\agents"
@"
---
name: xdg-test
description: "Test agent to verify XDG_CONFIG_HOME support."
---
Reply with OK.
"@ | Set-Content "$env:XDG_CONFIG_HOME\.copilot\agents\xdg-test.agent.md"
```

3. Launch Copilot CLI and check `/agents`

```text
/agent

Custom Agents

Select Agent:
❯ 1. Default (current)

Manage Agents:
1. Create new agent...
2. Learn more about custom agents

↑↓ to navigate · Enter to select · Esc to cancel
```

### Expected behavior

xdg-test appears in the list of available agents:

```
Custom Agents

Select Agent:
❯ 1. Default (current)
2. xdg-test · user

Manage Agents:
3. Create new agent...
4. Learn more about custom agents

↑↓ to navigate · Enter to select · Esc to cancel
```

### Additional context

_No response_