Make universal default prompt plain

[JonasHelming]

fixed #15006

What it does

• Make the default prompt of universal agent plain, just with the user context.
• Extract prompt to separate file (in preparation of licensing all prompts under MIT)

How to test

Talk to universal with default and empty prompt, it should behave llike the underlying LLM. The default should respect given context, e.g. "Add documentation to this source code file:filepath" shoudl work. This does not work with the empty variant.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[planger]

Works fine and looks good to me. Thank you!