Use setSessionAgent (first-write wins) instead of updateSessionAgent in chat.message handler. This prevents the default agent from overwriting a custom agent that was set via UI switch. Fixes #893 Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode) Co-authored-by: justsisyphus <justsisyphus@users.noreply.github.com> Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>