feat: add model tracking and reporting across pipeline

- Track actual model name from router through audit logs, session.json, and query output
- Add router-utils.ts to resolve model names from ROUTER_DEFAULT env var
- Inject model info into final report's Executive Summary section
- Update documentation with supported providers, pricing, and config examples
- Update router-config.json with latest model versions (GPT-5.2, Gemini 2.5, etc.)

src/audit/audit-session.ts

@@ -26,9 +26,10 @@ interface AgentEndResult {
   duration_ms: number;
   cost_usd: number;
   success: boolean;
-  error?: string;
-  checkpoint?: string;
-  isFinalAttempt?: boolean;
+  model?: string | undefined;
+  error?: string | undefined;
+  checkpoint?: string | undefined;
+  isFinalAttempt?: boolean | undefined;
 }
 
 /**