fix: use session's original createdAt instead of current time

Fixed bug where audit system would create duplicate folders for the same
session because it was using current time instead of the session's original
createdAt timestamp.

Bug behavior:
- Session created at T1 → folder: {T1}_app_host_id/
- Audit re-initialized at T2 → NEW folder: {T2}_app_host_id/
- Result: 2 folders per session with same ID but different timestamps

Root cause:
- metrics-tracker.js:65 was calling formatTimestamp() (current time)
- Should use sessionMetadata.createdAt (original creation time)

Impact: Each running benchmark was creating 2 audit log folders instead of 1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/audit/metrics-tracker.js

@@ -62,7 +62,7 @@ export class MetricsTracker {
         webUrl: this.sessionMetadata.webUrl,
         repoPath: this.sessionMetadata.repoPath,
         status: 'in-progress',
-        createdAt: formatTimestamp()
+        createdAt: this.sessionMetadata.createdAt || formatTimestamp()
       },
       metrics: {
         total_duration_ms: 0,