Add Assistant that utilizes the Claude Agent SDK

[mattlindsey]

Allow creation of a Claude Code Assistant. This may take modified (or a different) API Service and Language Model Settings pages.
Initial goal is the ability to send a single one-shot message to Claude Code and receive a reply.

[mattlindsey]

This task is 'on hold' since there might be a separate 'engine' project being developed that could be utilized to do the work.