Slack gives no sign that the bot received a message or is writing #852

A person who writes to the bot in Slack gets no sign that the message arrived until the reply lands. The first reply takes 21 seconds at the median on one install, but 133 seconds at the 90th percentile and up to 288 seconds (sandbox start plus tool work); later turns reached 411 seconds. People tag the bot again or assume it is down.

The Slack channel (chat/channels/slack/channel.py) has no acknowledgement: no reaction, no placeholder, no status. The GitHub QA pattern of adding 👀 on receipt does not exist for Slack.

Expected

• On receipt, the channel adds a reaction to the person's message (reactions.add, 👀), and replaces it when the reply is posted (✅). The bot needs the reactions:write scope.
• While the agent works, the channel posts a placeholder reply and updates it with the agent's notes so far (chat.update, every couple of seconds). The final update replaces the notes with the answer only, the text that Send a chat channel only the text after the agent's last tool call #922 sends. A bot can edit its own message, and the rate limit allows this cadence. follow_turn already streams the agent's events to the web page, so the Slack channel can consume the same stream.
• GitHub keeps posting one comment at the end; a comment edited live would notify on every change.