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/merge-descriptors/index.d.ts

@@ -0,0 +1,11 @@
+/**
+Merges "own" properties from a source to a destination object, including non-enumerable and accessor-defined properties. It retains original values and descriptors, ensuring the destination receives a complete and accurate copy of the source's properties.
+
+@param destination - The object to receive properties.
+@param source - The object providing properties.
+@param overwrite - Optional boolean to control overwriting of existing properties. Defaults to true.
+@returns The modified destination object.
+*/
+declare function mergeDescriptors<T, U>(destination: T, source: U, overwrite?: boolean): T & U;
+
+export = mergeDescriptors;