Some opinionated ideas and refactoring

[akirakyle]

Since I live in org mode and heavily use babel for literate programming I was excited to use it to integrate llm workflows into emacs in a way that's very familiar to me. As I was playing around with getting this set up, I was hitting some snags trying to get exactly requests I wanted. Over the course of trying to understand the code I ended up refactoring a fair bit to try to reason about the complex interactions between all the ways to set prompts, contexts, and generally all the gptel-* variables that implicitly affect a gptel-request. Anyways I thought I would just open this as a draft PR to get some visibility in case others are experimenting with this type of workflow and interested in any of these changes. A summary of what's here (or at least what I intended to be here):

• trying to make the babel src block arguments take precedence (either adding or overriding) over the global or per-buffer options that can be set from gptel-menu or in one's init
• removed the markdown to org conversion as I believe the models understand org mode markup fairly well and can output it semi-correctly if prompted correctly. With the output contained in a results drawer, org mode should do a reasonable-ish job of making sure any broken markup is contained
• added a :media header option and changed the gptel src block to org-mode so that org mode file links can have their contents included in a request if set to t
• Added tool use using changes from PR Add tool-use support to enable LLM function calling #7
• Removed the broken variable assignment code as I wasn't sure exactly what the usecase or ui should look like