Feature request: stash prompt

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

Ability to stash current prompt text for retrieval later

### Proposed solution

The following seems to happen regularly; I take time to craft a long prompt with various details that take a while to get. I then want to do another action in Copilot CLI; such as change models with `/model`, enable keep-alive (`/keep-alive`), change sessions, execute some bash command to find something, `/diff` to see if there are other changes I should commit first, etc.

Currently I rely on either copy/paste or opening a new terminal session for this.

It would be much more convenient if there was a way to stash my current prompt, do my additional actions, then retrieve my stashed prompt to execute or continue editing it.

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by tracing how the CLI reads and edits the current prompt and how commands such as /model, /keep-alive, and /diff are handled; done means a user can stash a prompt, perform those actions, and retrieve the prompt for continued editing or execution.

Scritto dal modello di indicizzazione a partire dal testo della issue.