Background agents cannot be stopped after user requests to stop

Problem

When a user explicitly instructs Claude Code to stop all work (e.g., "멈춰", "스톱", "멈추라고"), background agents launched with run_in_background: true continue running and consuming tokens. There is no mechanism to cancel or terminate running background agents.

Steps to Reproduce

1. Launch multiple background agents using run_in_background: true
2. User says "stop" / "멈춰"
3. Main agent acknowledges and stops launching new work
4. However, already-running background agents continue executing until completion
5. Tokens continue to be consumed against the user's explicit wishes

Expected Behavior

When the user instructs to stop, there should be a way to cancel/terminate running background agents immediately to prevent further token consumption.

Actual Behavior

Background agents run to completion regardless of user's stop request. The main agent has no tool or mechanism to cancel them. This results in unwanted token consumption and cost to the user.

Impact

• Unwanted token/cost consumption after explicit user stop request
• User frustration — feels like the system is ignoring their instructions
• No workaround available

Environment

• Claude Code VSCode Extension
• Windows 11
• Model: Claude Opus 4.6