fix: add base path for GitHub Pages deployment

Assets 404 without /hatk/ base when served from hatk-dev.github.io/hatk/.
Remove this once hatk.dev custom domain is active.

Co-Authored-By: Claude Opus 4.6 <[e-mail]>

docs/site/.vitepress/config.ts

···
 import { defineConfig } from 'vitepress'
 
 export default defineConfig({
+  base: '/hatk/',
   title: 'hatk',
   description: 'Build AT Protocol applications with typed XRPC endpoints.',