Only system comments allow HTML. There are no plans to provide full HTML support for regular users. There are no plans to allow for a WYSIWYG editor as well.

A comment flagged as being a system comment will allow HTML. It's a flag visible in backend and on frontend when creating a comment if the comment stream allows system comments. It's also available in the comment create of the CB Activity auto action type just set it to 1 and it'll mark the comment as a system comment and you can then use HTML in it. As for the system user that's configured globally within the CB Activity plugin.

It has nothing to do with a trigger. It's set on comment create. I guess you could flip a comment to a system comment with that trigger if you wanted, but it's not how you'd schedule creating comments. To schedule creating comments you'd just call your auto action from a CRON task to run it regularly on whatever schedule you want. Example as follows.

Global
Triggers: None
Type: CB Activity
User: Self
Access: Everybody
Action
Mode: Comment
Method: Create
Published: 1
Asset: activity.ACTIVITY_ID_HERE
Message: HTML_HERE
Owner: OWNING_USER_ID_HERE
System: 1
Load By: None

This will create a comment on the activity entry with an id matching ACTIVITY_ID_HERE and render it as a system comment supporting HTML.