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/tools/policy.ts

@@ -132,6 +132,14 @@ export interface ToolPolicyContext {
   agent?: string;
   /** Provider name (e.g. 'ollama', 'anthropic'). */
   provider?: string;
+  /** Session ID for audit logging. */
+  sessionId?: string;
+  /** Channel name for audit logging. */
+  channel?: string;
+  /** Sender ID for audit logging. */
+  sender?: string;
+  /** Model tier for audit logging. */
+  tier?: string;
 }
 
 // ── ToolPolicy engine ───────────────────────────────────────────────