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/is-promise/index.js

@@ -0,0 +1,6 @@
+module.exports = isPromise;
+module.exports.default = isPromise;
+
+function isPromise(obj) {
+  return !!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function';
+}