[BUG] Ctrl+Z keybinding behavior is inconsistent between running Claude Code inside and outside a tmux session

● Environment

• Platform (select one):
• [x] Anthropic API
• Claude CLI version: 1.0.44 (Claude Code)
• Operating System: Pop!_OS 22.04 (Linux 6.12.10)
• Terminal: tmux 3.2a

Bug Description

Ctrl+Z keybinding behavior is inconsistent between running Claude Code inside and outside a tmux session. Inside tmux, Ctrl+Z acts as "undo" instead of suspending Claude Code.

Steps to Reproduce

1. Open a terminal without tmux and run claude
2. Press Ctrl+Z - Claude Code suspends correctly (SIGTSTP)
3. Type fg to resume, then exit Claude Code
4. Start a tmux session with tmux
5. Inside tmux, run claude
6. Press Ctrl+Z - Claude Code performs "undo" action instead of suspending

Expected Behavior

Ctrl+Z should suspend Claude Code with SIGTSTP signal consistently, whether running inside or outside a tmux session.

Actual Behavior

• Outside tmux: Ctrl+Z suspends Claude Code (correct behavior)
• Inside tmux: Ctrl+Z acts as "undo" command (same as Ctrl+U)

Additional Context

Both Ctrl+Z and Ctrl+U perform "undo" when Claude Code runs inside tmux. This prevents users from suspending Claude Code with the standard Unix Ctrl+Z shortcut when working in tmux sessions.