How do I select an agent provided by an installed Agent Plugin?

The `plugins:` documentation explains how to install an Agent Plugin, and the Copilot engine documentation explains how `engine.agent` selects a custom agent from `.github/agents/`. I could not find documentation connecting these two features.

For example, the C# .NET Development plugin includes an agent named `expert-dotnet-software-engineer`:

```yaml
engine:
id: copilot
# How do I select the agent here?

plugins:
- github/awesome-copilot/plugins/csharp-dotnet-development@6859615e539eb3f4826075fb6a6661dbab8f6d03
```

How should I configure this workflow so that `expert-dotnet-software-engineer` runs as the primary agent instead of the default Copilot agent?

Is the intended configuration:

```yaml
engine:
id: copilot
agent: expert-dotnet-software-engineer
```

Or does a plugin-provided agent require a qualified identifier, such as `csharp-dotnet-development:expert-dotnet-software-engineer`?

If plugin-provided agents are available only as sub-agents rather than as the primary agent, how should the workflow invoke one by name?

Could the Agent Plugins documentation include an example showing how to select or invoke an agent supplied by an installed plugin, and clarify how this relates to `engine.agent`, imported agent files, and inline sub-agents?

## References

- [Agent Plugins](https://github.github.com/gh-aw/reference/frontmatter/#agent-plugins-plugins)
- [Copilot custom agents](https://github.github.com/gh-aw/reference/copilot-custom-agents/)
- [Copilot engine custom-agent selection](https://github.github.com/gh-aw/reference/engines/#copilot-custom-configuration)
- [C# .NET Development plugin](https://github.com/github/awesome-copilot/tree/main/plugins/csharp-dotnet-development)
- [Expert .NET software engineer agent](https://github.com/github/awesome-copilot/blob/main/agents/expert-dotnet-software-engineer.agent.md)

Start with the Agent Plugins, Copilot custom agents, and Copilot engine custom-agent selection references, then inspect the linked C# .NET Development plugin and expert-dotnet-software-engineer agent. Document whether engine.agent accepts the plugin agent name, requires qualification, or supports only sub-agent invocation, with a working workflow example and clear behavior for each case.

Written by the indexing model from the issue text.