Option to keep history when delegating to background agent

[anthonykim1]

Testing #280451

I noticed when I delegate to background agent, the entire chat history gets taken away with that delegated background agent.
I am then left with empty chat.
Now wondering if there should be way for user to keep the history pre-delegation to background agent instead of always starting out in new chat after the delegation.

[joshspicer]

you have hit on a largely discussed topic :)

I added a setting today with #280747 to disable the 'auto close', however this will certainly go through more changes