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/atlas/index.ts

@@ -1,5 +1,6 @@
 export { ATLAS_SYSTEM_PROMPT, getDefaultAtlasPrompt } from "./default"
 export { ATLAS_GPT_SYSTEM_PROMPT, getGptAtlasPrompt } from "./gpt"
+export { ATLAS_GEMINI_SYSTEM_PROMPT, getGeminiAtlasPrompt } from "./gemini"
 export {
   getCategoryDescription,
   buildAgentSelectionSection,