Do not print 'START' messages in interactive mode

As far as I understand, there is infrastructure for bst to show a TUI-like list of currently running tasks at the bottom of the terminal. I think it would be nicer if that was used for all messages when in interactive mode and the "START doing X" message were skipped from the visible output.

Start at the bst interactive-mode output path and trace how the TUI-like list displays currently running tasks alongside visible messages. Confirm where the "START doing X" message is emitted, then verify that interactive mode omits it while other modes retain it and that the task list still shows the running work.

Written by the indexing model from the issue text.