tui: add --no-keyboard-enhancement for terminal compatibility

Summary

Add a TUI CLI flag to disable keyboard enhancement escape modes:

• --no-keyboard-enhancement

Why

Some terminals show broken behavior when keyboard enhancements are enabled. A concrete case is foot, where copy shortcuts like Ctrl+Shift+C can trigger unexpected focus/refocus behavior back to the input box.

A flag-based opt-out provides a minimal compatibility path without changing defaults for terminals where enhancements work correctly.

Proposed behavior

• New flag: codex --no-keyboard-enhancement
• Also supported via codex resume --no-keyboard-enhancement
• Default behavior remains unchanged (enhancements enabled unless explicitly disabled)

Ready implementation

I have a complete, tested implementation in my fork:

• Branch: https://github.com/wegel/codex/tree/pr/no-keyboard-enhancement
• Compare: https://github.com/openai/codex/compare/main...wegel:pr/no-keyboard-enhancement?expand=1
• Commit: 61a5d3b2ea9f86ece6cf608e1f06d823f82db38e

Validation run

• cargo test -p codex-cli resume_no_keyboard_enhancement_flag_sets_mode -- --nocapture
• Manual verification in foot: copy behavior works correctly with --no-keyboard-enhancement