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/@hono/node-server/dist/utils/response.mjs

@@ -0,0 +1,10 @@
+// src/utils/response/constants.ts
+var X_ALREADY_SENT = "x-hono-already-sent";
+
+// src/utils/response.ts
+var RESPONSE_ALREADY_SENT = new Response(null, {
+  headers: { [X_ALREADY_SENT]: "true" }
+});
+export {
+  RESPONSE_ALREADY_SENT
+};