This PR makes it so that when you install an agent from the registry we
set it as the default agent. We apply the same behaviour when installing
an ACP agent from the onboarding page, which should make it less
confusing for new users (they won't see the Zed agent when they see a
new thread, but instead see the agent they installed)



https://github.com/user-attachments/assets/81f4711f-4c9a-4606-882a-9f944c90dfa3



Release Notes:

- agent: Improved onboarding experience when installing ACP agents