feat(03-01): create MetricsCollector and wire into gateway

- Add MetricsCollector class with counters, model call ring buffer, event ring buffer, and active request tracking
- Add system.metrics, system.events, system.activeRequests RPC handlers
- Add GET /health unauthenticated HTTP endpoint for Docker HEALTHCHECK
- Add totalPending() to LaneQueue for queue depth metrics
- Add 20 tests for MetricsCollector

src/gateway/handlers/agent.ts

@@ -3,11 +3,13 @@ import type { SendFn } from '../router.js';
 import { makeEvent, makeError, ErrorCode } from '../protocol.js';
 import type { SessionBridge } from '../session-bridge.js';
 import type { LaneQueue } from '../lane-queue.js';
+import type { MetricsCollector } from '../metrics.js';
 import type { Attachment } from '../../channels/types.js';
 
 export interface AgentHandlerDeps {
   sessionBridge: SessionBridge;
   laneQueue: LaneQueue;
+  metrics?: MetricsCollector;
 }
 
 export function createAgentHandlers(deps: AgentHandlerDeps) {