Ctrl+Z suspends CLI — should be intercepted or remapped

Problem

Ctrl+Z sends SIGTSTP and suspends the Claude Code process. This is almost never intentional — users hit it expecting "undo" (the universal shortcut in every other application). Instead, the entire session freezes and drops to the shell with no warning.

Expected behavior

Ctrl+Z should be intercepted by the CLI and either:

• Ignored (with a brief flash message like "use exit to leave")
• Mapped to undo (undo last input character / line, consistent with every other text input on earth)

Why this matters

• Muscle memory makes this extremely common
• Recovering a suspended session (fg) isn't obvious to many users
• Mid-conversation suspends can cause confusion about session state
• On mobile terminals / tmux, fg behavior can be inconsistent

This is a small quality-of-life fix that would eliminate a frequent source of frustration.