Add audit logs and update gitignore for xben results

Updates .gitignore to only ignore top-level audit-logs/ directory, allowing xben-benchmark-results audit logs to be tracked. This enables full reproducibility of benchmark runs with complete session data, prompts, and agent execution logs.

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

.gitignore

@@ -1,4 +1,4 @@
 node_modules/
 .shannon-store.json
 agent-logs/
-audit-logs/
+/audit-logs/