Feature Request: Persistent Shell Buffer Support

Add an option to preserve the shell output buffer even after the window is closed, similar to persistent scrollback behavior found in modern terminal emulators.

This would allow users to reopen a shell session (or scroll within it) and still access previous output, which is especially useful when working with long-running commands, logs, or debugging output:

- Prevents accidental loss of important output

- Improves usability during scripting, troubleshooting, or demonstrations

- Makes the shell experience feel more robust and modern

The issue does not name files, tests, or an entry point. Begin by tracing how Multipass creates and closes shell sessions, then determine the expected persistence and reopening behavior before identifying tests that can define completion.

Written by the indexing model from the issue text.