Support handoffs in Agent mode

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

Copilot CLI doesn't support "handoffs" feature right now

### Proposed solution

For Copilot CLI, when an Agent is executing a specified task, if that Agent has defined `handoffs`, it would be great if an option could be provided at the end to manage those handoffs

### Example prompts or workflows

1. Use `/agent` switch to a `plan` agent.
2. Choose `plan` agent I assume.
3. Enter `Plan a Sudoku game`
4. Copilot CLI switch to `implement` agent automatically and put prompt in the input box.

if multiple prompt available, show selection.

### Additional context

_No response_