feat(prometheus): add GPT-5.2 optimized prompt with model-based routing

- Create gpt.ts with XML-tagged, principle-driven prompt (Codex plan mode style)
- Add getPrometheusPrompt() routing: GPT models → GPT prompt, others → default
- Promote gpt-5.2 (high) to #2 in prometheus fallback chain
- Follow Atlas GPT variant pattern (isGptModel detection)

src/agents/prometheus/system-prompt.ts

@@ -4,9 +4,11 @@ import { PROMETHEUS_PLAN_GENERATION } from "./plan-generation"
 import { PROMETHEUS_HIGH_ACCURACY_MODE } from "./high-accuracy-mode"
 import { PROMETHEUS_PLAN_TEMPLATE } from "./plan-template"
 import { PROMETHEUS_BEHAVIORAL_SUMMARY } from "./behavioral-summary"
+import { getGptPrometheusPrompt } from "./gpt"
+import { isGptModel } from "../types"
 
 /**
- * Combined Prometheus system prompt.
+ * Combined Prometheus system prompt (Claude-optimized, default).
  * Assembled from modular sections for maintainability.
  */
 export const PROMETHEUS_SYSTEM_PROMPT = `${PROMETHEUS_IDENTITY_CONSTRAINTS}
@@ -27,3 +29,32 @@ export const PROMETHEUS_PERMISSION = {
   webfetch: "allow" as const,
   question: "allow" as const,
 }
+
+export type PrometheusPromptSource = "default" | "gpt"
+
+/**
+ * Determines which Prometheus prompt to use based on model.
+ */
+export function getPrometheusPromptSource(model?: string): PrometheusPromptSource {
+  if (model && isGptModel(model)) {
+    return "gpt"
+  }
+  return "default"
+}
+
+/**
+ * Gets the appropriate Prometheus prompt based on model.
+ * GPT models → GPT-5.2 optimized prompt (XML-tagged, principle-driven)
+ * Default (Claude, etc.) → Claude-optimized prompt (modular sections)
+ */
+export function getPrometheusPrompt(model?: string): string {
+  const source = getPrometheusPromptSource(model)
+
+  switch (source) {
+    case "gpt":
+      return getGptPrometheusPrompt()
+    case "default":
+    default:
+      return PROMETHEUS_SYSTEM_PROMPT
+  }
+}