[BUG] Ctrl+B backgrounds running tool on single press despite UI showing double-press requirement

Bug Description

The UI prompt during tool execution shows Ctrl+B, Ctrl+B (double-press) to background a running task, but pressing Ctrl+B just once immediately backgrounds it.

Expected Behavior

Either:

• The double-press requirement shown in the UI should be enforced (requiring Ctrl+B, Ctrl+B), consistent with how Ctrl+F, Ctrl+F works for terminating background agents, OR
• The UI should be updated to show a single Ctrl+B if single-press is the intended behavior

Steps to Reproduce

1. Start a tool that takes a few seconds (e.g. a bash command)
2. Press Ctrl+B once
3. The task is immediately backgrounded, despite the UI indicating Ctrl+B, Ctrl+B is required

Environment

• OS: Linux (Ubuntu, kernel 6.8.0)
• Terminal: bash (no tmux/byobu/screen)
• Claude Code: latest