Resolve the Auto classifier spec and any reason it was ignored.
Reads the models.auto_classifier option from env/config.toml. None
means the classifier inherits the main agent model, which is the historical
behavior and the default.
A configured-but-unusable value (blank, or a non-string such as
auto_classifier = 3, which resolve_scalar drops to the default) silently
reverts authorization review to the main agent model — the agent grading its
own actions. The caller gets a description so it can say so on a surface the
user actually reads; a log line alone is not that surface.
