fn participating(replies: &[SlackHistoryMessage], user_id: &str) -> boolExpand description
Whether a thread is one the user is in, read from its replies: his own message, or one that names him.
He replies from the Slack app rather than through forge, so this is detected from the wire rather than remembered from forge’s own record. A mention counts, because being named in a thread is the other half of the spec’s rule.