fix: keep background exec aborts from killing sessions (#1000) (thanks @roshanasingh4)

When exec returns early in background mode, the tool-call AbortSignal can fire and previously caused killProcessTree(SIGKILL). Ignore abort after yielding/backgrounding so background sessions keep running.

src/agents/bash-tools.exec.ts

@@ -263,6 +263,7 @@ export function createExecTool(
       };
 
       const onAbort = () => {
+        if (yielded || session.backgrounded) return;
         killSession(session);
       };