The goal is to have a single place where we actually write files
In a follow-up PR, will move everything config related in a dedicated
module and move the helpers in a dedicated file
It turns out that we want slightly different behavior for the
`SetDefaultModel` RPC because some models do not work with reasoning
(like GPT-4.1), so we should be able to explicitly clear this value.
Verified in `codex-rs/mcp-server/tests/suite/set_default_model.rs`.
