feat(governance): add structured JSONL audit trail with SIEM-ready schema

New audit-trail.mjs writes structured events to LLM_SECURITY_AUDIT_LOG path.
Integrated into post-session-guard at 6 warning emission points: trifecta,
escalation-after-input, data flow, volume threshold, slow-burn, behavioral drift.
No-op when env var not set — zero overhead for existing users.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/llm-security/scanners/lib/audit-trail.mjs

@@ -0,0 +1,83 @@
+// audit-trail.mjs — Structured JSONL audit trail writer
+// Writes SIEM-ready events to the path specified by LLM_SECURITY_AUDIT_LOG.
+// No-op when env var is not set. Zero external dependencies.
+
+import { appendFileSync, writeFileSync, accessSync, constants } from 'node:fs';
+import { dirname } from 'node:path';
+
+let auditPath = null;
+let initialized = false;
+
+/**
+ * Initialize audit trail. Validates the path is writable on first call.
+ * @returns {boolean} true if audit trail is enabled and writable
+ */
+function initAuditTrail() {
+  if (initialized) return auditPath !== null;
+  initialized = true;
+
+  const envPath = process.env.LLM_SECURITY_AUDIT_LOG;
+  if (!envPath) return false;
+
+  try {
+    // Ensure parent directory exists and is writable
+    const dir = dirname(envPath);
+    accessSync(dir, constants.W_OK);
+    // Touch file if it doesn't exist
+    try { accessSync(envPath); } catch { writeFileSync(envPath, ''); }
+    auditPath = envPath;
+    return true;
+  } catch (err) {
+    process.stderr.write(`[llm-security] Audit trail path not writable: ${envPath} (${err.message})\n`);
+    return false;
+  }
+}
+
+/**
+ * Write a structured audit event as one JSON line.
+ * No-op when LLM_SECURITY_AUDIT_LOG is not set.
+ *
+ * @param {object} event
+ * @param {string} event.event_type - e.g. trifecta_warning, injection_detected
+ * @param {string} event.severity - critical|high|medium|low|info
+ * @param {string} event.source - hook or scanner name
+ * @param {object} [event.details] - event-specific payload
+ * @param {string[]} [event.owasp] - OWASP categories
+ * @param {string} [event.action_taken] - blocked|warned|allowed
+ */
+export function writeAuditEvent(event) {
+  if (!initAuditTrail()) return;
+
+  const entry = {
+    timestamp: new Date().toISOString(),
+    session_id: String(process.ppid || process.pid),
+    event_type: event.event_type || 'unknown',
+    severity: event.severity || 'info',
+    source: event.source || 'unknown',
+    details: event.details || {},
+    owasp: event.owasp || [],
+    action_taken: event.action_taken || 'warned',
+  };
+
+  try {
+    appendFileSync(auditPath, JSON.stringify(entry) + '\n');
+  } catch (err) {
+    process.stderr.write(`[llm-security] Audit trail write failed: ${err.message}\n`);
+  }
+}
+
+/**
+ * Check whether audit trail is enabled (for guard clauses in hooks).
+ * @returns {boolean}
+ */
+export function isAuditEnabled() {
+  return initAuditTrail();
+}
+
+/**
+ * Reset internal state (for testing only).
+ */
+export function _resetForTest() {
+  auditPath = null;
+  initialized = false;
+}