Custom agent as default agent (auto load agent on new copilot-cli session)

### Describe the feature or problem you'd like to solve

On starting a new session (copilot-cli), i need to /agent and then select my custom agent.

### Proposed solution

Custom agent as default agent (auto load agent on new copilot-cli session), like the same way you have the model. I see them being a part of the config.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start at the copilot-cli new-session startup flow and review how the /agent selection and model configuration are handled. Define how a custom agent could be stored in config and loaded automatically when a session starts; done means a configured custom agent is selected without running /agent.

索引モデルが issue の本文から書いたものです。