feat(routing): add pi canary guardrails and backend audit telemetry

src/audit/logger.ts

@@ -23,6 +23,7 @@ import type {
   UserActionEvent,
   QueuePreemptEvent,
   BackendRouteEvent,
+  BackendSuccessEvent,
   BackendFallbackEvent,
   CronTriggerEvent,
   WebhookReceiveEvent,
@@ -215,6 +216,11 @@ export class AuditLogger {
     this.write({ level: 'info', event_type: 'backend.route', event: event as unknown as Record<string, unknown> });
   }
 
+  backendSuccess(event: BackendSuccessEvent): void {
+    if (!this.shouldLog('sessions', 'info')) {return;}
+    this.write({ level: 'info', event_type: 'backend.success', event: event as unknown as Record<string, unknown> });
+  }
+
   backendFallback(event: BackendFallbackEvent): void {
     if (!this.shouldLog('sessions', 'warn')) {return;}
     this.write({ level: 'warn', event_type: 'backend.fallback', event: event as unknown as Record<string, unknown> });

src/audit/types.ts

@@ -12,7 +12,7 @@ export type AuditEventType =
   // Session lifecycle
   | 'session.create' | 'session.message' | 'session.delete' | 'session.transfer' | 'session.compact' | 'session.checkpoint' | 'session.auto_compact' | 'user.action'
   | 'queue.preempt'
-  | 'backend.route' | 'backend.fallback'
+  | 'backend.route' | 'backend.success' | 'backend.fallback'
   // Automation - Cron
   | 'cron.trigger' | 'cron.sent' | 'cron.add' | 'cron.remove'
   // Automation - Webhook
@@ -236,17 +236,28 @@ export interface BackendRouteEvent {
   session_id: string;
   channel: string;
   sender: string;
-  selected_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini';
-  source: 'agent_override' | 'default_external' | 'native';
+  selected_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini' | 'pi_embedded';
+  source: 'agent_override' | 'default_external' | 'native' | 'forced_native_guard';
+  guard_reason?: 'capability_query' | 'pi_no_tools_mode' | 'attachments_present';
+}
+
+export interface BackendSuccessEvent {
+  session_id: string;
+  channel: string;
+  sender: string;
+  backend: 'claude_code' | 'opencode' | 'codex' | 'gemini' | 'pi_embedded';
+  duration_ms: number;
+  response_length: number;
 }
 
 export interface BackendFallbackEvent {
   session_id: string;
   channel: string;
   sender: string;
-  from_backend: 'claude_code' | 'opencode' | 'codex' | 'gemini';
-  to_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini';
+  from_backend: 'claude_code' | 'opencode' | 'codex' | 'gemini' | 'pi_embedded';
+  to_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini' | 'pi_embedded';
   reason: string;
+  duration_ms?: number;
 }
 
 export interface CronTriggerEvent {