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/backends/native/orchestrator.ts

@@ -12,6 +12,7 @@ import type { OutboundAttachmentCollector } from './attachments.js';
 import { shouldCompact } from '../../context/tokens.js';
 import { compactHistory, type CompactionConfig, type CompactionResult, DEFAULT_COMPACTION_CONFIG } from '../../context/compaction.js';
 import { estimateCost } from '../../models/costs.js';
+import { auditLogger } from '../../audit/index.js';
 
 // ── Public types ──────────────────────────────────────────────────────
 
@@ -255,6 +256,16 @@ export class AgentOrchestrator {
       `${result.tokensBefore} → ${result.tokensAfter} tokens`,
     );
 
+    if (this._session) {
+      auditLogger?.sessionCompact({
+        session_id: this._session.id,
+        messages_before: result.tokensBefore,
+        messages_after: result.tokensAfter,
+        tokens_before: result.tokensBefore,
+        tokens_after: result.tokensAfter,
+      });
+    }
+
     return result;
   }