Allow enabling web_search in Azure DevOps repositories

> Drafted with GitHub Copilot CLI based on observed behavior and local testing.

## Feature request: allow enabling `web_search` in Azure DevOps repositories

### Summary

When Copilot CLI is started from an Azure DevOps-only repository, the built-in GitHub MCP tooling appears to be unavailable. As a result, `web_search` is also unavailable, even when I only want web search and do not want GitHub repository tools.

I could not find a documented CLI flag, hook, or setting that lets me override this behavior.

### Why this matters

`web_search` is useful independently of where the current repository is hosted. In Azure DevOps repositories, I still want Copilot CLI to be able to look up documentation, investigate errors, research packages, and gather general engineering context from the web.

### Current behavior

Starting Copilot CLI in an Azure DevOps-only repository makes `web_search` unavailable.

Passing:

```bash
copilot --add-github-mcp-tool web_search
```

does not appear to enable it in this case. My understanding is that this option can limit which GitHub MCP tools are exposed, but it does not force the underlying built-in MCP server to be available when Copilot CLI has decided not to load it.

Similarly, enabling the server via MCP settings does not appear to provide a reliable documented way to override this startup behavior.

### Expected behavior

There should be a supported way to explicitly enable `web_search` in non-GitHub repositories, for example:

```bash
copilot --add-github-mcp-tool web_search
```

or:

```bash
copilot --enable-mcp-server github-mcp-server --add-github-mcp-tool web_search
```

or an equivalent setting.

The important part is that an explicit user choice should be able to override the automatic repository-host-based default.

### Workaround

A workaround seems to be starting Copilot CLI outside the Azure DevOps repository, with:

```bash
copilot --add-github-mcp-tool web_search
```

and then changing directories into the Azure DevOps repo from inside the session.

That workaround is fragile and non-obvious, so it would be better to have a documented and supported configuration option.

### Suggested behavior

The default behavior can remain conservative: if Copilot CLI wants to hide GitHub repository tools in Azure DevOps repositories, that makes sense.

But users should be able to explicitly opt in to a narrow tool set, especially:

```bash
web_search
```

without also enabling unrelated GitHub repository operations.