Workspace MCP servers should take precedence over plugins

### Describe the bug

I'm developing an MCP server that I also have installed as a plugin. Copilot CLI uses `/.mcp.json` in a repo root - which I can tell from renaming the MCP server - but if it retains the same name, the plugin is what shows up with `/mcp`. Seems the repository should always take precedence, shouldn't it? Skills do, as is evident from `/skills` - project skills with the same name show "(project)" instead of "(plugin)". This is inconsistent behavior with MCP servers.

### Affected version

0.0.422-0

### Steps to reproduce the behavior

1. Install a plugin with an MCP server.
2. Add a `.mcp.json` file to your repo root with the same name and configuration as that MCP server.
3. Start copilot and run `/mcp`.

### Expected behavior

Should show something like,

```
Workspace:
> ✓ example-mcp Description of MCP server
```

Instead, shows it from a plugin:

```
Plugins:
> ✓ example-mcp Description of MCP server
```

### Additional context

- macOS 26.3
- Ghostty 1.2.3
- bash 5.3.9(1)-release (aarch64-apple-darwin25.1.0)