feat(sisyphus-junior): add model-specific GPT prompt routing (gpt-5-4, gpt-5-3-codex, generic gpt)

Split GPT prompt into three variants with model-based routing:
- gpt-5-4.ts: GPT-5.4 optimized (expert coding agent framing, prose-first)
- gpt-5-3-codex.ts: GPT-5.3-Codex optimized (Hephaestus-style Senior Engineer)
- gpt.ts: Generic GPT fallback (Hephaestus-style, for any other GPT model)

Routing: gpt-5.4 → gpt-5-4 | gpt-5.3-codex → gpt-5-3-codex | other GPT → gpt

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/sisyphus-junior/agent.ts

@@ -21,6 +21,8 @@ import {
 
 import { buildDefaultSisyphusJuniorPrompt } from "./default"
 import { buildGptSisyphusJuniorPrompt } from "./gpt"
+import { buildGpt54SisyphusJuniorPrompt } from "./gpt-5-4"
+import { buildGpt53CodexSisyphusJuniorPrompt } from "./gpt-5-3-codex"
 import { buildGeminiSisyphusJuniorPrompt } from "./gemini"
 
 const MODE: AgentMode = "subagent"
@@ -34,13 +36,13 @@ export const SISYPHUS_JUNIOR_DEFAULTS = {
   temperature: 0.1,
 } as const
 
-export type SisyphusJuniorPromptSource = "default" | "gpt" | "gemini"
+export type SisyphusJuniorPromptSource = "default" | "gpt" | "gpt-5-4" | "gpt-5-3-codex" | "gemini"
 
-/**
- * Determines which Sisyphus-Junior prompt to use based on model.
- */
 export function getSisyphusJuniorPromptSource(model?: string): SisyphusJuniorPromptSource {
   if (model && isGptModel(model)) {
+    const lower = model.toLowerCase()
+    if (lower.includes("gpt-5.4") || lower.includes("gpt-5-4")) return "gpt-5-4"
+    if (lower.includes("gpt-5.3-codex") || lower.includes("gpt-5-3-codex")) return "gpt-5-3-codex"
     return "gpt"
   }
   if (model && isGeminiModel(model)) {
@@ -60,6 +62,10 @@ export function buildSisyphusJuniorPrompt(
   const source = getSisyphusJuniorPromptSource(model)
 
   switch (source) {
+    case "gpt-5-4":
+      return buildGpt54SisyphusJuniorPrompt(useTaskSystem, promptAppend)
+    case "gpt-5-3-codex":
+      return buildGpt53CodexSisyphusJuniorPrompt(useTaskSystem, promptAppend)
     case "gpt":
       return buildGptSisyphusJuniorPrompt(useTaskSystem, promptAppend)
     case "gemini":