feat(ci): add PR preview deployments

- Add pr-preview.yml workflow using rossjrw/pr-preview-action
- Update astro.config.mjs to support dynamic base path via ASTRO_BASE env
- Previews deploy to /pr-preview/pr-<number>/ on gh-pages branch
- Auto-cleanup when PR is closed

website/astro.config.mjs

@@ -1,10 +1,13 @@
 import sitemap from "@astrojs/sitemap";
 import { defineConfig } from "astro/config";
 
+// Support dynamic base path for PR previews via ASTRO_BASE env var
+const base = process.env.ASTRO_BASE || "/";
+
 // https://astro.build/config
 export default defineConfig({
   site: "https://github.github.io",
-  base: "/",
+  base: base,
   output: "static",
   integrations: [sitemap()],
   build: {