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/pkce-challenge/dist/index.browser.d.ts

@@ -0,0 +1,19 @@
+/** Generate a PKCE code challenge from a code verifier
+ * @param code_verifier
+ * @returns The base64 url encoded code challenge
+ */
+export declare function generateChallenge(code_verifier: string): Promise<string>;
+/** Generate a PKCE challenge pair
+ * @param length Length of the verifer (between 43-128). Defaults to 43.
+ * @returns PKCE challenge pair
+ */
+export default function pkceChallenge(length?: number): Promise<{
+    code_verifier: string;
+    code_challenge: string;
+}>;
+/** Verify that a code_verifier produces the expected code challenge
+ * @param code_verifier
+ * @param expectedChallenge The code challenge to verify
+ * @returns True if challenges are equal. False otherwise.
+ */
+export declare function verifyChallenge(code_verifier: string, expectedChallenge: string): Promise<boolean>;