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/config/schema/agent-names.ts

@@ -11,6 +11,7 @@ export const BuiltinAgentNameSchema = z.enum([
   "metis",
   "momus",
   "atlas",
+  "sisyphus-junior",
 ])
 
 export const BuiltinSkillNameSchema = z.enum([