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/session/manager.ts

@@ -1,5 +1,6 @@
 import type { Message } from '../models/types.js';
 import type { SessionStore } from './store.js';
+import { auditLogger } from '../audit/index.js';
 
 export interface Session {
   id: string;
@@ -23,6 +24,15 @@ export class ManagedSession implements Session {
     };
     this.history.push(messageWithTimestamp);
     this.store.addMessage(this.id, messageWithTimestamp);
+    
+    auditLogger?.sessionMessage({
+      session_id: this.id,
+      role: message.role,
+      content_length: typeof message.content === 'string' 
+        ? message.content.length 
+        : JSON.stringify(message.content).length,
+    });
+    
     return messageWithTimestamp;
   }
 
@@ -31,8 +41,14 @@ export class ManagedSession implements Session {
   }
 
   clear(): void {
+    const messageCount = this.history.length;
     this.history = [];
     this.store.clearSession(this.id);
+    
+    auditLogger?.sessionDelete({
+      session_id: this.id,
+      message_count: messageCount,
+    });
   }
 
   /**
@@ -67,6 +83,12 @@ export class SessionManager {
       const history = this.store.getMessages(id);
       session = new ManagedSession(id, this.store, history);
       this.sessions.set(id, session);
+      
+      auditLogger?.sessionCreate({
+        session_id: id,
+        frontend,
+        user_id: userId,
+      });
     }
 
     return session;
@@ -88,6 +110,8 @@ export class SessionManager {
     for (const message of history) {
       toSession.addMessage(message);
     }
+    
+    auditLogger?.sessionTransfer(fromSession.id, toSession.id, history.length);
   }
 
   listSessions(): string[] {