feat(API-11): Route API calls through AgilitonAPI gateway

Add gateway-first pattern: when AGILITON_API_KEY is set, route all
external API calls through the gateway with X-API-Key auth. Falls back
to direct API access when gateway is unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

node_modules/zod/v3/index.d.cts

@@ -0,0 +1,4 @@
+import * as z from "./external.cjs";
+export * from "./external.cjs";
+export { z };
+export default z;