Change incorrect object format model.primary/fallbacks to correct
flat format model_name/model_fallbacks in Agent defaults example.
The AgentDefaults struct does not support the object format used
in AgentConfig, so the documentation example was misleading.
* refactor: support explicit model list providers
* fix(web): preserve explicit model providers
* fix(web): preserve legacy provider prefixes on model updates
fix(models): normalize explicit provider-prefixed ids
fix(api): preserve legacy model updates across providers
fix(agent): preserve config identity for explicit provider refs
* fix ci
