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/index.ts

@@ -14,6 +14,10 @@ export { createAtlasAgent, atlasPromptMetadata } from "./atlas"
 export {
   PROMETHEUS_SYSTEM_PROMPT,
   PROMETHEUS_PERMISSION,
+  PROMETHEUS_GPT_SYSTEM_PROMPT,
+  getPrometheusPrompt,
+  getPrometheusPromptSource,
+  getGptPrometheusPrompt,
   PROMETHEUS_IDENTITY_CONSTRAINTS,
   PROMETHEUS_INTERVIEW_MODE,
   PROMETHEUS_PLAN_GENERATION,
@@ -21,3 +25,4 @@ export {
   PROMETHEUS_PLAN_TEMPLATE,
   PROMETHEUS_BEHAVIORAL_SUMMARY,
 } from "./prometheus"
+export type { PrometheusPromptSource } from "./prometheus"