Previously, applyModelDefaults would return early if cfg.agents.defaults.models
was empty, which meant the DEFAULT_MODEL_ALIASES (opus, sonnet, gpt, etc.)
were never added to the config. This caused isolated cron sessions to fail
with 'Unknown model' errors when using aliases like 'sonnet'.
Now we always iterate through DEFAULT_MODEL_ALIASES and add them to the
config, creating model entries if they don't exist.
