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/shared/model-requirements.ts

@@ -64,9 +64,9 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   prometheus: {
     fallbackChain: [
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-opus-4-6", variant: "max" },
+      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.2", variant: "high" },
       { providers: ["kimi-for-coding"], model: "k2p5" },
       { providers: ["opencode"], model: "kimi-k2.5-free" },
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.2", variant: "high" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-pro" },
     ],
   },