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/index.ts

@@ -1,4 +1,11 @@
-export { PROMETHEUS_SYSTEM_PROMPT, PROMETHEUS_PERMISSION } from "./system-prompt"
+export {
+  PROMETHEUS_SYSTEM_PROMPT,
+  PROMETHEUS_PERMISSION,
+  getPrometheusPrompt,
+  getPrometheusPromptSource,
+} from "./system-prompt"
+export type { PrometheusPromptSource } from "./system-prompt"
+export { PROMETHEUS_GPT_SYSTEM_PROMPT, getGptPrometheusPrompt } from "./gpt"
 
 // Re-export individual sections for granular access
 export { PROMETHEUS_IDENTITY_CONSTRAINTS } from "./identity-constraints"