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.

.github/workflows/npm-publish.yml

@@ -16,9 +16,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup Bun
-        uses: oven-sh/setup-bun@v2
-
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
@@ -32,7 +29,7 @@ jobs:
         run: npm install -g npm@latest
 
       - name: Install dependencies
-        run: bun install --frozen-lockfile
+        run: npm ci
 
       - name: Publish
         run: npm publish --provenance --access public