execute: stop streaming output to agent after process detach

When a process is detached, nil out lw.stream so output no longer
leaks to the agent's chat stream. Only the ring buffer captures
subsequent output.

pkg/tools/execute/server.go

@@ -748,6 +748,7 @@ func (e *Server) executeWithStdin(ctx context.Context, code, language string, ti
 		// Splice: future output goes to ring buffer instead of outputBuf.
 		lw.mu.Lock()
 		lw.w = ring
+		lw.stream = nil
 		lw.mu.Unlock()
 
 		proc := &DetachedProcess{