refactor: cut over to the Astro build

Merges refactor/task-20-cutover. Task 20 steps 1, 2, and 5; publishing is
not included.

The hand-written site is gone: 32 files deleted, including app.js,
responsive.css, and all ten route index.html files. Twelve more could not
be deleted -- the Astro pages import them and the build fails without
them -- so they moved to legacy/ verbatim, outside the reach of
check-tokens.mjs, which sweeps src/ and would demand a token migration
these files have not had.

Before anything was deleted, rendered-text-diff swept all ten routes plus
both Portuguese pages at full parity, 0 missing and 0 extra. That
comparison stops being possible once the legacy files are gone, which is
why it ran first. computed-style-diff on /full-guide/ is unchanged at 32.

verify.mjs no longer reads app.js and holds at 84 assertions.
audit-ui.mjs reads dist/. Docs across README, AGENTS.md, GATES.md, the
architecture context, and the operations guide now describe the built
site rather than the hand-written one.

origin/pages is unchanged at 37a1e480c6.
The publish job is still gated to manual dispatch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/audit-ui.mjs

@@ -1,18 +1,20 @@
 import { readFileSync, readdirSync, statSync } from 'node:fs';
 import { dirname, join } from 'node:path';
 
+const base = '/ai-for-dummies/';
 const read = (path) => readFileSync(new URL(`../${path}`, import.meta.url), 'utf8');
+// The legacy hand-written pages are gone; the built output is the site now.
 const pages = [
-  'index.html',
-  'full-guide/index.html',
-  'summary/index.html',
-  'models/index.html',
-  'agents/index.html',
-  'skills/index.html',
-  'rules/index.html',
-  'skills-review/index.html',
-  'hands-on/starter/index.html',
-  'hands-on/rules/index.html',
+  'dist/index.html',
+  'dist/full-guide/index.html',
+  'dist/summary/index.html',
+  'dist/models/index.html',
+  'dist/agents/index.html',
+  'dist/skills/index.html',
+  'dist/rules/index.html',
+  'dist/skills-review/index.html',
+  'dist/hands-on/starter/index.html',
+  'dist/hands-on/rules/index.html',
 ];
 const walk = (path) =>
   readdirSync(path).flatMap((name) => {
@@ -61,8 +63,13 @@ for (const [kind, values] of Object.entries(baselineValues)) {
   const missing = values.filter((value) => !builtValues[kind].has(value));
   if (missing.length) throw new Error(`built CSS lost ${kind}: ${missing.join(', ')}`);
 }
-// Legacy fixtures still ship directly. Resolve every var() from the stylesheets
-// that page actually links; Astro's tokens cannot mask a broken standalone page.
+// The hands-on labs still ship their own stylesheet rather than going through
+// Astro. Resolve every var() from the stylesheets each page actually links, so
+// Astro's tokens cannot mask a broken standalone page.
+// Astro emits base-absolute hrefs (`/ai-for-dummies/_astro/x.css`); those are
+// rooted at `dist/`, not at the page's own directory.
+const resolveHref = (page, href) =>
+  href.startsWith(base) ? join('dist', href.slice(base.length)) : join(dirname(page), href);
 for (const page of pages) {
   const html = read(page);
   const linked = [...html.matchAll(/<link[^>]+rel="stylesheet"[^>]+href="([^"]+)"/gi)].map(
@@ -70,7 +77,7 @@ for (const page of pages) {
   );
   const styles = linked
     .filter((href) => !/^https?:/i.test(href))
-    .map((href) => read(join(dirname(page), href)));
+    .map((href) => read(resolveHref(page, href)));
   const defined = new Set(styles.flatMap((css) => [...definitions(css)]));
   const unresolved = new Set(
     styles.flatMap((css) =>