Allow disabling GitHub Copilot as an Agent Host provider without disabling Chat

VS Code should provide a setting that disables the GitHub Copilot Agent Host provider without disabling the Chat feature or other Agent Host providers.

A setting consistent with the existing provider settings would be:

```jsonc
{
"chat.agentHost.copilotAgent.enabled": false
}
```

The existing settings provide this control for Claude and Codex:

```jsonc
{
"chat.agentHost.claudeAgent.enabled": true,
"chat.agentHost.codexAgent.enabled": true
}
```

There is currently no equivalent setting for Copilot.

## Problem

I use the Codex Agent Host provider with my OpenAI subscription. I do not use GitHub Copilot models.

I have hidden all Copilot models in Manage Language Models. However, Copilot still appears in Chat and the model selector still displays unavailable Copilot models, sign-in prompts, and upgrade actions.

Model visibility therefore does not disable the Copilot provider itself.

The existing alternatives do not solve this:

- `chat.disableAIFeatures` disables the complete Chat feature, including providers I want to use.
- Hiding Copilot models does not remove the provider or unavailable featured models.
- Disabling Copilot autocomplete-related features does not remove the Agent Host provider.
- There is no `chat.agentHost.copilotAgent.enabled` equivalent to the Claude and Codex settings.
- The former global `chat.agentHost.enabled` setting was removed and would have been too broad in any case.

## Requested behavior

When `chat.agentHost.copilotAgent.enabled` is `false`:

1. Do not register or advertise the Copilot/Copilot CLI Agent Host provider.
2. Do not offer Copilot as a provider or harness for new Chat sessions.
3. Remove Copilot-hosted models from Chat model selectors.
4. Do not display Copilot-specific unavailable models, sign-in prompts, upgrade actions, quota messages, or model promotions in Chat.
5. Keep the general Chat feature enabled.
6. Keep Codex, Claude, BYOK, and third-party Agent Host providers available.
7. Preserve existing Copilot sessions without deleting user data. They can remain readable when explicitly opened, but Copilot should not be offered for new sessions.
8. Preserve the current behavior when the setting is absent or `true`.

Requiring a window reload after changing the setting would be acceptable if provider registration occurs during Agent Host startup.

## Scope

This request targets the Chat and Agent Host provider system.

The following features are out of scope and can continue to use their existing controls:

- Inline completions
- Next-edit suggestions
- Commit-message generation
- Code review
- Other Copilot features that are not implemented through Agent Host

This request also does not ask for a global opt-out from promotions for every model provider. It only asks that Copilot-specific UI disappear when the Copilot provider is explicitly disabled.

## Steps demonstrating the current behavior

1. Enable the Codex Agent Host provider.
2. Open Manage Language Models.
3. Hide every GitHub Copilot model.
4. Open Chat and its model or provider selector.
5. Observe that Copilot-related entries and upgrade affordances are still present.
6. Try to disable only the Copilot provider.
7. Observe that no supported provider-level setting exists.

## Related work

- [#321155](https://github.com/microsoft/vscode/pull/321155) added `chat.agentHost.claudeAgent.enabled` and `chat.agentHost.codexAgent.enabled`, but no Copilot equivalent.
- [#331464](https://github.com/microsoft/vscode/issues/331464) notes that no per-provider Copilot toggle currently exists, but that issue primarily concerns authentication.
- [#320213](https://github.com/microsoft/vscode/issues/320213) requested hiding upgrade-only Copilot models, but was closed without implementation.
- [#330934](https://github.com/microsoft/vscode/issues/330934) requests a broader opt-out from model promotional notifications.
- [#329303](https://github.com/microsoft/vscode/pull/329303) removed the global Agent Host rollout setting, making provider-specific controls more important.