Add undo support (Ctrl+_ / Cmd+Z) to input line editor

## Summary

The Copilot CLI input line editor currently lacks undo functionality. Standard readline/terminal behavior supports `Ctrl+_` for undo, but this doesn't work in Copilot CLI because the CLI implements its own input
handling.

## Current behavior

- `Ctrl+_` does nothing in Copilot CLI
- No undo capability exists for typed input
- Other standard readline shortcuts work (Ctrl+a, Ctrl+e, Ctrl+w, Ctrl+u, Ctrl+k)

## Expected behavior

- `Ctrl+_` should undo the last edit (standard readline undo)
- Ideally maintain an undo history for multiple undo operations

## Use case

When typing longer prompts, accidentally deleting text or making typos requires retyping from scratch. Undo support is a standard text editing feature that would significantly improve the input experience.

## Environment

- macOS + iTerm2
- Copilot CLI version: 0.0.377
- Shell: zsh

## Additional context

`Ctrl+_` works correctly in a clean terminal (readline handles it), confirming this is specific to Copilot CLI's custom input handling.