feat(audit): Add core audit logging infrastructure

- Add AuditLogger class with rotation support
- Add audit configuration to config schema
- Instrument tool execution with full audit logging
- Instrument session lifecycle (create, message, delete, transfer, compact)
- Add audit logger initialization in daemon
- Add cron scheduler audit logging

Audit events captured:
- tool.start/success/error/denied
- session.create/message/delete/transfer/compact
- cron.trigger/add/remove

All logs go to ~/.local/share/flynn/audit.log (JSON lines)
with rotation (10MB files, 30-day retention)

src/audit/index.ts

@@ -0,0 +1,9 @@
+import type { AuditLogger } from './logger.js';
+export * from './types.js';
+export { AuditLogger } from './logger.js';
+
+export let auditLogger: AuditLogger | null = null;
+
+export function initAuditLogger(logger: AuditLogger): void {
+  auditLogger = logger;
+}