refactor(runtime): replace Bun dependency with Node fetch

Run Perplexity search and auth requests through pi's Node runtime instead of
shelling out to Bun. Move development and CI commands to npm, add a Node test
build path, and keep OTP auth fail-fast when Set-Cookie headers are not
available.

test/perplexity-fetch.test.ts

@@ -0,0 +1,56 @@
+import { afterEach, describe, expect, mock, test } from "./test-helpers.js";
+
+import { perplexityFetchText } from "../src/perplexity-fetch.js";
+
+const originalFetch = globalThis.fetch;
+const originalGetSetCookie = (Headers.prototype as Headers & { getSetCookie?: () => string[] }).getSetCookie;
+
+afterEach(() => {
+  globalThis.fetch = originalFetch;
+  Object.defineProperty(Headers.prototype, "getSetCookie", {
+    configurable: true,
+    writable: true,
+    value: originalGetSetCookie,
+  });
+});
+
+describe("perplexityFetchText", () => {
+  test("returns all Set-Cookie headers from Node fetch", async () => {
+    globalThis.fetch = mock(async () =>
+      new Response("ok", {
+        status: 200,
+        headers: [
+          ["set-cookie", "first=1; Path=/; HttpOnly"],
+          ["set-cookie", "second=2; Path=/; Secure"],
+        ],
+      }),
+    ) as unknown as typeof fetch;
+
+    const response = await perplexityFetchText("https://example.com", {
+      method: "GET",
+      headers: {},
+    });
+
+    expect(response.ok).toBe(true);
+    expect(response.status).toBe(200);
+    expect(response.bodyText).toBe("ok");
+    expect(response.cookies).toEqual([
+      "first=1; Path=/; HttpOnly",
+      "second=2; Path=/; Secure",
+    ]);
+  });
+
+  test("fails when the Node fetch runtime cannot expose Set-Cookie headers", async () => {
+    Object.defineProperty(Headers.prototype, "getSetCookie", {
+      configurable: true,
+      writable: true,
+      value: undefined,
+    });
+
+    globalThis.fetch = mock(async () => new Response("ok", { status: 200 })) as unknown as typeof fetch;
+
+    await expect(perplexityFetchText("https://example.com", { method: "GET", headers: {} })).rejects.toThrow(
+      "Headers.getSetCookie",
+    );
+  });
+});