refactor(sisyphus): extract prompt builders into subdirectory with GPT-5.4 variant

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/sisyphus/index.ts

@@ -0,0 +1,19 @@
+/**
+ * Sisyphus agent — multi-model orchestrator.
+ *
+ * This directory contains model-specific prompt variants:
+ * - default.ts: Base implementation for Claude and general models
+ * - gemini.ts: Corrective overlays for Gemini's aggressive tendencies
+ * - gpt-5-4.ts: Native GPT-5.4 prompt with block-structured guidance
+ */
+
+export { buildDefaultSisyphusPrompt, buildTaskManagementSection } from "./default";
+export {
+  buildGeminiToolMandate,
+  buildGeminiDelegationOverride,
+  buildGeminiVerificationOverride,
+  buildGeminiIntentGateEnforcement,
+  buildGeminiToolGuide,
+  buildGeminiToolCallExamples,
+} from "./gemini";
+export { buildGpt54SisyphusPrompt } from "./gpt-5-4";