fix(#338): Add max concurrent agents limit

- Add MAX_CONCURRENT_AGENTS configuration (default: 20)
- Check current agent count before spawning
- Reject spawn requests with 429 Too Many Requests when limit reached
- Add comprehensive tests for limit enforcement

Refs #338

apps/orchestrator/src/config/orchestrator.config.ts

@@ -37,4 +37,7 @@ export const orchestratorConfig = registerAs("orchestrator", () => ({
   yolo: {
     enabled: process.env.YOLO_MODE === "true",
   },
+  spawner: {
+    maxConcurrentAgents: parseInt(process.env.MAX_CONCURRENT_AGENTS ?? "20", 10),
+  },
 }));