Enable plugins to override the model and provider for agent runs by
returning modelOverride/providerOverride from the before_agent_start
hook. The hook is now invoked early in run.ts (before resolveModel)
so overrides take effect. The result is passed to attempt.ts via
earlyHookResult to prevent double-firing.
This enables security-critical use cases like routing PII-containing
prompts to local models instead of cloud providers.