Proposal: Provide a single prompt field when configuring an agent

Problem to solve

The current agent configuration splits the system and user prompts into separate fields. We've gotten feedback that this is confusing. It's also unnecessary as the system prompt is sufficient to pass instructions to the model.

Proposal

1. Remove the User prompt input from the agent configuration page.
2. Have the system prompt input support lengthy text input. This could be done by having the multi-line input expand to fit the content as it's typed (example).

Current example