bug: the model config in custom agent does not work.

https://forgecode.dev/docs/agent-definition-guide/#complete-configuration-options

<img width="915" height="598" alt="Image" src="https://github.com/user-attachments/assets/e11eb3d8-9e22-4486-91b7-0096600ffead" />

It just not implemented.

https://github.com/antinomyhq/forge/pull/1386