Bring back `profile =` toml key instead of duplicating config

Each profile is supposed to be in its own file. So one should not be duplicating default in the main toml file. That in-turn means you have to pass --profile everytime one wants to run codex.

Sometimes I want to run multiple sessions using a different model. The removal of default profile option worsens the UX. It's stupid to pass the cmdline flag everytime.

(And the override files weren't even put in a config.toml.d like standard unix design.)