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/builtin-agents/general-agents.ts

@@ -50,6 +50,7 @@ export function collectPendingBuiltinAgents(input: {
     if (agentName === "sisyphus") continue
     if (agentName === "hephaestus") continue
     if (agentName === "atlas") continue
+    if (agentName === "sisyphus-junior") continue
     if (disabledAgents.some((name) => name.toLowerCase() === agentName.toLowerCase())) continue
 
     const override = agentOverrides[agentName]