Flexible configuration for submit and new line actions

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

A way to define what combinations do a submit or a new line

### Proposed solution

Currently it is possible to configure SHIFT+ENTER as new line but ENTER always stays as submit.
More tools are allowing or adopting the CMD+ENTER as submit to avoid accidental submits.
Having this possibility in copilot would help avoiding accident submits.

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files or tests; start by tracing the existing SHIFT+ENTER new-line configuration and ENTER submit handling in the CLI input path. Define how submit and new-line combinations should be configured, then add coverage showing the selected combinations work without accidental submits.

Written by the indexing model from the issue text.