fix(agent): initialize with config model value

[Tibiritabara]

Currently, the model initialized in the agent, is taking a default value from args despite the user customization on the toml or the env vars.

This PR just replaces that line, to take the value from the config instead.

[rbren]

This will break the frontend--it prevents the user from setting the model in the frontend.

I realize this workflow is kind of confusing as-is--we're working on making it better