feat: model config selection should be persisted

[rekram1-node]

When I change my model selection and exit opencode, the selection should be persisted the next time I open opencode again.

I imagine this would be the global config file that is updated but I wonder if people would like the ability to have selection persisted per project.

Main questions:

• Should it be persisted globally?
• Should we just persist the coding agent? < I imagine so since we don't allow selection of the title or task agents