Add origin=(step, ) for root messages with *

[abitrolly]

This separates log semantic and representation.

Right now the first line printed with _ctx.log gets bold is prefixed '* ', and subsequent lines in the same message are indented. This makes things more complicated than necessary. It is more convenient to just print step messages bold and prefixed, and plain _ctx.log(message) as plain text.

[abitrolly]

I propose to merge main logger functionality into _ctx. The _ctx default logger is only used in tests, and its behavior is different. Tests can mock logging if needed as everything else.

I also propose to replace origin tuple with plain string for readability.