Ctrl-B doesn't move cursor left in Chat input

Description

In the Chat input area, Ctrl-B does not move the cursor left one character as expected with standard readline/Emacs keybindings.

Expected behavior

Ctrl-B should move the cursor one character to the left (backward-char), matching readline behavior.

Actual behavior

Nothing happens when pressing Ctrl-B.

Notes

• Left arrow key works correctly
• Other readline keybindings appear to work
• Ctrl-B is not bound in ~/.claude/keybindings.json (Chat context or otherwise), so it should pass through to the input handler
• Not using tmux or anything else that would capture Ctrl-B

Environment

• macOS (Darwin 25.2.0)