Make terminals ephemeral per turn

[meganrogge]

Now that sessions aren't archived when one clicks +, one can have many terminals hanging around potentially. Since we retain their output, we should consider clearing them per turn.

Request from @hawkticehurst and @pierceboggan

[meganrogge]

@Tyriar had the good pt that this could cause issues if ppl are working in parallel. Env and stuff could be reset.

[meganrogge]

He mentioned we could kill background terminals that are finished since those aren't reused.

[meganrogge]

We should also make sure that the terminal is still hidden in this case, not revealed by the user