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/tests/lib/audit-trail.test.mjs

@@ -0,0 +1,109 @@
+// audit-trail.test.mjs — Tests for structured JSONL audit trail
+
+import { describe, it, beforeEach, afterEach } from 'node:test';
+import assert from 'node:assert/strict';
+import { writeFileSync, readFileSync, unlinkSync, existsSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+import { writeAuditEvent, isAuditEnabled, _resetForTest } from '../../scanners/lib/audit-trail.mjs';
+
+const TEST_LOG = join(tmpdir(), `llm-security-audit-test-${Date.now()}.jsonl`);
+
+describe('audit-trail', () => {
+  beforeEach(() => {
+    _resetForTest();
+    // Clean up test file
+    try { unlinkSync(TEST_LOG); } catch {}
+  });
+
+  afterEach(() => {
+    _resetForTest();
+    delete process.env.LLM_SECURITY_AUDIT_LOG;
+    try { unlinkSync(TEST_LOG); } catch {}
+  });
+
+  it('is disabled when env var not set', () => {
+    delete process.env.LLM_SECURITY_AUDIT_LOG;
+    assert.equal(isAuditEnabled(), false);
+  });
+
+  it('is enabled when env var is set to writable path', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = TEST_LOG;
+    assert.equal(isAuditEnabled(), true);
+  });
+
+  it('no-op when env var not set', () => {
+    delete process.env.LLM_SECURITY_AUDIT_LOG;
+    writeAuditEvent({ event_type: 'test', severity: 'info', source: 'test' });
+    assert.equal(existsSync(TEST_LOG), false);
+  });
+
+  it('writes valid JSONL when enabled', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = TEST_LOG;
+    writeAuditEvent({
+      event_type: 'trifecta_warning',
+      severity: 'high',
+      source: 'post-session-guard',
+      details: { window_size: 20 },
+      owasp: ['ASI01', 'ASI02'],
+      action_taken: 'warned',
+    });
+
+    const content = readFileSync(TEST_LOG, 'utf8').trim();
+    const entry = JSON.parse(content);
+
+    assert.equal(entry.event_type, 'trifecta_warning');
+    assert.equal(entry.severity, 'high');
+    assert.equal(entry.source, 'post-session-guard');
+    assert.deepEqual(entry.owasp, ['ASI01', 'ASI02']);
+    assert.equal(entry.action_taken, 'warned');
+    assert.ok(entry.timestamp.match(/^\d{4}-\d{2}-\d{2}T/), 'Expected ISO timestamp');
+    assert.ok(entry.session_id, 'Expected session_id');
+  });
+
+  it('appends multiple events as separate lines', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = TEST_LOG;
+    writeAuditEvent({ event_type: 'event1', severity: 'info', source: 'test' });
+    writeAuditEvent({ event_type: 'event2', severity: 'medium', source: 'test' });
+    writeAuditEvent({ event_type: 'event3', severity: 'high', source: 'test' });
+
+    const lines = readFileSync(TEST_LOG, 'utf8').trim().split('\n');
+    assert.equal(lines.length, 3);
+
+    const e1 = JSON.parse(lines[0]);
+    const e3 = JSON.parse(lines[2]);
+    assert.equal(e1.event_type, 'event1');
+    assert.equal(e3.event_type, 'event3');
+  });
+
+  it('events contain all required fields', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = TEST_LOG;
+    writeAuditEvent({ event_type: 'test', severity: 'info', source: 'test-hook' });
+
+    const entry = JSON.parse(readFileSync(TEST_LOG, 'utf8').trim());
+    const required = ['timestamp', 'session_id', 'event_type', 'severity', 'source', 'details', 'owasp', 'action_taken'];
+    for (const field of required) {
+      assert.ok(field in entry, `Missing required field: ${field}`);
+    }
+  });
+
+  it('provides defaults for optional fields', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = TEST_LOG;
+    writeAuditEvent({ event_type: 'minimal' });
+
+    const entry = JSON.parse(readFileSync(TEST_LOG, 'utf8').trim());
+    assert.equal(entry.severity, 'info');
+    assert.equal(entry.source, 'unknown');
+    assert.deepEqual(entry.details, {});
+    assert.deepEqual(entry.owasp, []);
+    assert.equal(entry.action_taken, 'warned');
+  });
+
+  it('does not crash on invalid path', () => {
+    process.env.LLM_SECURITY_AUDIT_LOG = '/nonexistent/dir/audit.jsonl';
+    // Should not throw — gracefully logs to stderr
+    assert.doesNotThrow(() => {
+      writeAuditEvent({ event_type: 'test', severity: 'info', source: 'test' });
+    });
+  });
+});