Add name and system prompt to AI Agent chat interface

The following discussion from !141698 (merged) should be addressed:

• @msaleiko started a discussion: (+5 comments)

@marshall007 @msaleiko in the screenshots you have the system prompt pinned at the top, but this was no longer present in the version demo'd today. Should we add it back?

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@marshall007 that's true. But because I rebased on the recent work that Darby and Eduardo did (especially the Vue router update), we need to fetch that data with a GraphQl query. Before I added the prompt as a data attribute to the standalone Vue app. That's no longer possible.

I'd want to have the prompt in the show agent view, so I'll look into fetching the data via GraphQl.

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@eduardobonet adding the prompt could be done in a follow up, it does not need to be part of this MR. I think it's ok to fetch the entire prompt from the backend, it isn't different than fetching an issue description.

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@msaleiko Yes I think so, too. This MR should be as short as possible. We can iterate on everything else after the initial merge of the chat interface.