refactor(agent-config): use getAgentListDisplayName for consistent naming

Update agent config handler and tests to use the new getAgentListDisplayName

utility for proper OpenCode agent list display ordering.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>