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

[willhasgithub]

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.