fix: register sisyphus-junior as builtin agent across type system and model fallback

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.

Closes code-yeongyu/oh-my-openagent#1697

src/agents/types.ts

@@ -113,7 +113,8 @@ export type BuiltinAgentName =
   | "multimodal-looker"
   | "metis"
   | "momus"
-  | "atlas";
+  | "atlas"
+  | "sisyphus-junior";
 
 export type OverridableAgentName = "build" | BuiltinAgentName;