- add athena to built-in and overridable agent name schemas
- extend BuiltinAgentName with athena for config-level recognition
- make builtin agent source maps partial until athena runtime registration lands
Sisyphus-Junior was missing from BuiltinAgentName type, agentSources map,
barrel exports, and AGENT_MODEL_REQUIREMENTS. This caused type inconsistencies
and prevented model-fallback hooks from working for sisyphus-junior sessions.
Closescode-yeongyu/oh-my-openagent#1697