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/index.ts

@@ -2,3 +2,4 @@ export * from "./types"
 export { createBuiltinAgents } from "./builtin-agents"
 export type { AvailableAgent, AvailableCategory, AvailableSkill } from "./dynamic-agent-prompt-builder"
 export type { PrometheusPromptSource } from "./prometheus"
+export { createSisyphusJuniorAgentWithOverrides, SISYPHUS_JUNIOR_DEFAULTS } from "./sisyphus-junior"