/export command uses name "Agent" for Jupyternaut's messages

[JasonWeill]

When I use the /export command (see #658), the name used for Jupyternaut's messages is "Agent", not "Jupyternaut". If we let users change the default character's name, or create additional characters for AI assistance, this will be ambiguous. We should use the same character name in the chat panel and in exported files.

**Agent**: Hi there! I'm Jupyternaut, your programming assistant.
You can ask me a question using the text box below. You can also use these commands:
* `/ask` — Ask a question about your learned data
* `/clear` — Clear the chat window
* `/generate` — Generate a Jupyter notebook from a text prompt
* `/learn` — Teach Jupyternaut about files on your system
* `/export` — Export chat history to a Markdown file
* `/fix` — Fix an error cell selected in your notebook
* `/help` — Display this help message

Jupyter AI includes [magic commands](https://jupyter-ai.readthedocs.io/en/latest/users/index.html#the-ai-and-ai-magic-commands) that you can use in your notebooks.
For more information, see the [documentation](https://jupyter-ai.readthedocs.io).


**jweill**: /help