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/config.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, test, beforeEach, afterEach } from "bun:test";
+import { afterEach, beforeEach, describe, expect, test } from "./test-helpers.js";
 import { mkdtemp, readFile, rm, stat, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -17,7 +17,7 @@ beforeEach(async () => {
   tempDir = await mkdtemp(join(tmpdir(), "pi-perplexity-test-"));
   configPath = join(tempDir, "config.json");
 
-  const mod = await import(`../src/config.ts?t=${Date.now()}`);
+  const mod = await import(`../src/config.js?t=${Date.now()}`);
   loadConfig = mod.loadConfig;
   saveConfig = mod.saveConfig;
   resolveSearchDefaults = mod.resolveSearchDefaults;