Nested .github/copilot/settings.json does not override enabledPlugins when cwd is below git root (worked in version 1.0.60)

### Describe the bug

When starting Copilot CLI from a nested directory below the git root, a local`.github/copilot/settings.json` in that nested cwd is not honored for `enabledPlugins`. A globally enabled marketplace plugin still loads even when the cwd-local settings file disables it.

### Affected version

GitHub Copilot CLI 1.0.61.

### Steps to reproduce the behavior

Directory layout:

```
repo/.git/ module/
.github/
copilot/
settings.json
```

Global ~/.copilot/settings.json:

```
...
{
"enabledPlugins": {
"example-plugin@example-marketplace": true
}
}
...
```

Nested repo/module/.github/copilot/settings.json:

```
{
"enabledPlugins": {
"example-plugin@example-marketplace": false
}
}
```

From repo/module, run:

copilot plugin list

### Expected behavior

The cwd-local .github/copilot/settings.json should override global settings,
and example-plugin@example-marketplace should be disabled for that session.

But the plugin still appears enabled/loaded from the global setting.

## Notes

The nested directory is not the git root. The git root is repo/.
If the same override is placed at `repo/.github/copilot/settings.json`, it is honored.

### Additional context

Abililty to keep settings.json at nested path under .git repo, gives us more flexibility. This was supported till version 1.0.60!

Start with the `copilot plugin list` entry point and trace how global, git-root, and cwd-local `.github/copilot/settings.json` files are discovered and merged. Reproduce the layout from the issue using `enabledPlugins`, then verify that a nested cwd setting disabling the plugin overrides the global setting while the git-root case continues to work.

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