Shift+Enter currently submits the prompt instead of inserting a new line

### Describe the bug

Not sure whether this is intended behavior or a feature gap, but it would be helpful if Shift+Enter inserted a new line instead of submitting the prompt to the agent.
Currently, if I press Shift+Enter while still composing a prompt, the agent immediately starts processing an unfinished input. This is frustrating, as it often forces me to cancel and re-enter the full prompt.

### Affected version

GitHub Copilot CLI 1.0.31

### Steps to reproduce the behavior

_No response_

### Expected behavior

Shift+Enter should create a new line while composing a prompt.

### Additional context

_No response_

No file or test is named. Start by running GitHub Copilot CLI 1.0.31 and tracing the prompt input and key-handling entry points for Shift+Enter; done means it inserts a newline while composing instead of submitting the prompt, without changing ordinary Enter behavior.

Written by the indexing model from the issue text.