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/math-intrinsics/sign.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var $isNaN = require('./isNaN');
+
+/** @type {import('./sign')} */
+module.exports = function sign(number) {
+	if ($isNaN(number) || number === 0) {
+		return number;
+	}
+	return number < 0 ? -1 : +1;
+};