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/shared/model-requirements.ts

@@ -157,6 +157,19 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
       { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.4", variant: "medium" },
     ],
   },
+  "sisyphus-junior": {
+    fallbackChain: [
+      {
+        providers: ["anthropic", "github-copilot", "opencode"],
+        model: "claude-sonnet-4-6",
+      },
+      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.4", variant: "medium" },
+      {
+        providers: ["google", "github-copilot", "opencode"],
+        model: "gemini-3-flash",
+      },
+    ],
+  },
 };
 
 export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {