feat(agents): wire Gemini prompt routing into Sisyphus-Junior, Atlas, Prometheus

Add 'gemini' to prompt source types and route Gemini models to new
Gemini-optimized prompts via isGeminiModel detection. Update barrel
exports for all 3 agent modules. All existing tests pass.

src/agents/sisyphus-junior/index.ts

@@ -1,5 +1,6 @@
 export { buildDefaultSisyphusJuniorPrompt } from "./default"
 export { buildGptSisyphusJuniorPrompt } from "./gpt"
+export { buildGeminiSisyphusJuniorPrompt } from "./gemini"
 
 export {
   SISYPHUS_JUNIOR_DEFAULTS,