Preview walk results now land in Slack with screenshots

Every time a pull request’s preview is walked, the result is posted to #build-log. Each pull request gets one thread. Each walk replies in that thread with the pass or fail verdict, any failures by persona, a dashboard screenshot for each of the six personas, and links to the full recording.

A failed walk is posted the same way as a passing one. If Slack refuses part of the post, the evidence still goes out in a simpler form and #alerts is told why.