refactor: consolidate duplicate types and file I/O utilities

- Remove 4 duplicate file I/O functions from audit/utils.ts, re-export from utils/file-io.ts
- Consolidate AgentEndResult interface into new types/audit.ts
- Use exported AgentDefinition from types/agents.ts in session-manager.ts
- Rename AgentMetrics to AgentAuditMetrics to disambiguate from temporal/shared.ts

src/audit/audit-session.ts

@@ -17,21 +17,11 @@ import { MetricsTracker } from './metrics-tracker.js';
 import { initializeAuditStructure, type SessionMetadata } from './utils.js';
 import { formatTimestamp } from '../utils/formatting.js';
 import { SessionMutex } from '../utils/concurrency.js';
+import type { AgentEndResult } from '../types/index.js';
 
 // Global mutex instance
 const sessionMutex = new SessionMutex();
 
-interface AgentEndResult {
-  attemptNumber: number;
-  duration_ms: number;
-  cost_usd: number;
-  success: boolean;
-  model?: string | undefined;
-  error?: string | undefined;
-  checkpoint?: string | undefined;
-  isFinalAttempt?: boolean | undefined;
-}
-
 /**
  * AuditSession - Main audit system facade
  */