refactor: use sayStream helper instead of client.chatStream

[zimeg]

Summary

Replace manual client.chatStream() calls with the sayStream context utility, which automatically injects channel, thread_ts, recipient_team_id, and recipient_user_id from the listener context.

export const message = async ({ logger, message, say, sayStream, setStatus }) => {
  await setStatus({ status: 'thinking...' });
  const streamer = sayStream({ task_display_mode: 'plan' });
  await streamer.append({ chunks: [{ type: 'markdown_text', text: 'Hello!' }] });
  await streamer.stop();
};

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.