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.ts

@@ -12,6 +12,7 @@ import { createMetisAgent, metisPromptMetadata } from "./metis"
 import { createAtlasAgent, atlasPromptMetadata } from "./atlas"
 import { createMomusAgent, momusPromptMetadata } from "./momus"
 import { createHephaestusAgent } from "./hephaestus"
+import { createSisyphusJuniorAgentWithOverrides } from "./sisyphus-junior"
 import type { AvailableCategory } from "./dynamic-agent-prompt-builder"
 import {
   fetchAvailableModels,
@@ -41,6 +42,7 @@ const agentSources: Record<BuiltinAgentName, AgentSource> = {
   // Note: Atlas is handled specially in createBuiltinAgents()
   // because it needs OrchestratorContext, not just a model string
   atlas: createAtlasAgent as AgentFactory,
+  "sisyphus-junior": createSisyphusJuniorAgentWithOverrides as unknown as AgentFactory,
 }
 
 /**