Stash (ctrl+s) should integrate with prompt history instead of single-slot storage

Current Behavior

ctrl + s stashes the current input to a single slot. If you stash again before retrieving, it overwrites the previous stash.

Suggested Improvement

Instead of a separate single-slot stash, add the stashed input to the prompt history (accessible via up/down arrow keys).

Why This Would Be Better

1. Multiple stashes for free - History already supports multiple entries
2. Familiar pattern - Users already know up/down arrow from shell history
3. More discoverable - People instinctively try arrow keys
4. Simpler mental model - One history mechanism instead of two separate systems

Potential Concern

The current design might intentionally separate submitted prompts (history) from unsubmitted drafts (stash). But in practice, the convenience of a unified approach likely outweighs this distinction.

Reproduction

1. Type something, press ctrl + s to stash
2. Type something else, press ctrl + s again
3. Previous stash is lost (overwritten)