XML-style prompting

[ErikBjare]

Markdown has some pretty rough limitations that become especially apparent once you need to nest it.

A large class of bugs and workarounds could be eliminated if we switched to XML-style prompting, but on the other hand I've invested so much work into getting the markdown stuff to work that I'm pretty happy with where it is at.

Relevant issues:

• Give agent ability to distinguish code samples from runnable code #67 (making tool calls explicit XML/JSON messages, instead of ambiguous might-be-an-example codeblocks, would fix this)

[ErikBjare]

Some preparatory work done in #113

[ErikBjare]

It can be enabled in the code, and works great for Anthropic, but disabled by default still, until we make it toggleable/configurable.