Feature request: Set default agent for projects

Now we have the agent structure, we should be able to set a default agent for a project. The project would then automatically inherit the system prompt, parameters and so on from the agent. It could also be still possible to add project-level config such as files.

At the moment, you have to either manually set the agent each chat within the project, or copy and paste the system message and so on from the agent to the project config.