fix(tui): restore question prompt key handling

[nexxeln]

Summary

• give the TUI question prompt its own keymap mode while mounted
• restore Enter/Escape/app.exit handling when another mode was already active
• add regression coverage for submit, reject, and app.exit paths

Tests

• bun test test/cli/tui/question-prompt.test.tsx
• bun typecheck
• pre-push: bun turbo typecheck