allow conversations to continue with `%%assist`

[rgbkrk]

This captures past assistance as well as errors so that ChatGPT has access to the full context of:

• code (user)
• errors (system)
• suggestions (assistant)
• outputs (system)

This PR closes #63 since now the %%assist magic uses Markdown output instead of set_next_input. It also closes #56 since the progress display is no more.

Bonus: hot swapping the models

• [x] Update tests
• [x] Think a little more on how best to capture past assistant messages (does it belong in GenaiMarkdown?) Created a separate PastAssistant singleton

[rgbkrk]

Interesting little prompt response I just got:

%%assist

What is your level of understanding of the messages between what comes from me (`role: user`) versus the runtime (`role: system`)?

As a coding assistant, I am programmed to understand the messages that start with %%assist as user input. All other messages are interpreted as output from the system.

[rgbkrk]

This is so rad for educational usage.