[BUG] Ctrl+Z suspend stopped working in 2.1.0

Description

Ctrl+Z no longer suspends Claude Code as of version 2.1.0. The process remains in the foreground instead of suspending.

Steps to Reproduce

1. Launch Claude Code 2.1.0+
2. Press Ctrl+Z
3. Process remains in foreground (expected: suspend and return to shell)

Environment

• OS: macOS (Darwin 25.0.0)
• Terminal: iTerm2
• Claude Code version: 2.1.0, 2.1.1, 2.1.2, 2.1.3 (all affected)
• Last working version: 2.0.9

Additional Context

• Ctrl+Z works correctly in other applications (e.g., emacs) in the same terminal session
• Terminal stty settings are correct (susp = ^Z)
• This is a regression from the fix implemented for issue #588