Bug Report: Alt+Backspace incorrectly closes question panels instead of deleting word

Description
When typing in the "Other" option field of ExitPlanMode or AskUserQuestion pan
els, pressing Alt+Backspace (or Alt+Delete) to delete a word backward unexpect
edly closes/interrupts the question panel instead of performing the word delet
ion.
Steps to Reproduce

1. Start kimi-cli and enter plan mode
2. Make the AI generate a plan that requires user input (e.g., using ExitPlanM
   with options)
3. When the question panel appears, select the "Other" option to enter custom
   t
4. Type some text (e.g., "this is a test")
5. Press Alt+Backspace to delete the last word
   Expected Behavior
   The last word ("test") should be deleted, leaving "this is a ".
   Actual Behavior
   The entire question panel closes/interrupts, and the conversation is cancelled
   .
   Environment
   • OS: Linux (Ubuntu 24.04)
   • Terminal: tmux/Ghostty
   • kimi-cli version: (latest)
   Technical Details
   The issue is in kimi_cli/ui/shell/prompt.py. The Alt+Backspace key combination
   sends an escape sequence (ESC + Backspace). The code has an eager key binding
   for the escape key that exits the question panel, which intercepts the first p
   art of the sequence before the backspace is processed.
   Suggested Fix: Add key bindings for escape backspace and escape delete sequenc
   es that call backward_kill_word() when the question panel is in text input mod
   e. These bindings need to be registered before the simple escape binding to ta
   ke precedence.

Start in kimi_cli/ui/shell/prompt.py and inspect the eager Escape binding and the question panel's text-input mode. Reproduce the issue in kimi-cli using an Other option, then verify that Alt+Backspace and Alt+Delete delete a word without closing or cancelling the panel, while plain Escape still exits it.