feat: add bun-fullstack agent and update skills

.agent/skills/tech-stack/elysiajs/integrations/vercel.md

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+# Vercel Integration
+Deploy Elysia on Vercel
+
+## What It Is
+Zero-config deployment on Vercel (Bun or Node runtime).
+
+## Setup
+1. Create/import Elysia server in `src/index.ts`
+2. Export as default:
+```typescript
+import { Elysia, t } from 'elysia'
+
+export default new Elysia()
+  .get('/', () => 'Hello Vercel Function')
+  .post('/', ({ body }) => body, {
+    body: t.Object({ name: t.String() })
+  })
+```
+
+3. Develop locally:
+```bash
+vc dev
+```
+
+4. Deploy:
+```bash
+vc deploy
+```
+
+## Node.js Runtime
+Set in `package.json`:
+```json
+{
+  "name": "elysia-app",
+  "type": "module"
+}
+```
+
+## Bun Runtime
+Set in `vercel.json`:
+```json
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "bunVersion": "1.x"
+}
+```
+
+## pnpm
+Manual install:
+```bash
+pnpm add @sinclair/typebox openapi-types
+```
+
+## Troubleshooting
+Vercel has zero config for Elysia. For additional config, see [Vercel docs](https://vercel.com/docs/frameworks/backend/elysia).