Generate /skills index + 21 auto-rendered skill detail pages

Ships the first new sub-page section. Every user-invocable skill now has
its own page at /skills/{id}, with the canonical SKILL.md body rendered
via marked. The index at /skills lists all 21 skills grouped by category.

Editorial wrappers are opt-in: if content/site/skills/{id}.md exists, it
renders above the canonical body (with a "The skill itself" divider).
All 21 pages currently ship with the auto-rendered body only; hand-written
wrappers land in the next few commits.

- scripts/lib/sub-pages-data.js: builds the data model. Reuses
  readSourceFiles() from lib/utils.js for skill content; parses the
  ANTIPATTERNS array out of src/detect-antipatterns.mjs; reads optional
  editorial wrappers from content/site/skills/*.md; validates that every
  user-invocable skill has a category entry (build fails loudly if not).
- scripts/build-sub-pages.js: orchestrator. Writes generated HTML into
  public/skills/*.html (gitignored). Called from both scripts/build.js
  (before buildStaticSite) and server/index.js (at module load) so dev
  and prod share the same generation code path.
- scripts/lib/render-page.js: new assetDepth parameter so generated
  pages one level deep under public/ use relative paths (../favicon.svg,
  ../css/sub-pages.css) that Bun's HTML loader can resolve on disk.
- scripts/build.js: pass generated files into Bun.build entrypoints;
  post-process to flatten build/public/* → build/* (Bun preserves the
  public/ prefix when entrypoints span multiple depths).
- server/index.js: generateSubPages() runs at module load; new routes
  /skills, /skills/:id, /anti-patterns, /tutorials, /tutorials/:slug
  serve the pre-generated files via Bun.file().
- public/css/sub-pages.css: adds sub-page layout shell, skills index
  grouped-list styling, skill detail header/meta chips/divider, collapsed
  <details> reference sections, and a .prose block for rendered markdown
  with editorial typography, code blocks, and inline code.

Verified: bun run build produces 26 HTML files (4 hand-authored + 22
generated), all flat under build/. Dev server returns 200 on /skills,
/skills/polish, /skills/impeccable, /skills/critique. Tests pass.

server/index.js

@@ -1,4 +1,6 @@
 import { serve, file } from "bun";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
 import homepage from "../public/index.html";
 import cheatsheet from "../public/cheatsheet.html";
 import gallery from "../public/gallery.html";
@@ -11,6 +13,29 @@ import {
   handleFileDownload,
   handleBundleDownload
 } from "./lib/api-handlers.js";
+import { generateSubPages } from "../scripts/build-sub-pages.js";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const ROOT_DIR = path.resolve(__dirname, "..");
+
+// Pre-generate sub-pages so dev + prod share the same output shape.
+console.log("📝 Generating sub-pages for dev server...");
+const { files: subPageFiles } = await generateSubPages(ROOT_DIR);
+console.log(`✓ Generated ${subPageFiles.length} sub-page(s)`);
+
+// Helper: serve a generated HTML file by absolute path, 404 if missing.
+async function serveGenerated(pagePath) {
+  const f = file(pagePath);
+  if (!(await f.exists())) return new Response("Not Found", { status: 404 });
+  return new Response(f, {
+    headers: {
+      "Content-Type": "text/html;charset=utf-8",
+      "X-Content-Type-Options": "nosniff",
+      "X-Frame-Options": "DENY",
+    },
+  });
+}
 
 const server = serve({
   port: process.env.PORT || 3000,
@@ -21,6 +46,19 @@ const server = serve({
     "/gallery": gallery,
     "/privacy": privacy,
 
+    // Generated sub-pages — served directly from the pre-generated files
+    "/skills": () => serveGenerated(path.join(ROOT_DIR, "public/skills/index.html")),
+    "/skills/:id": (req) => {
+      const id = req.params.id.replace(/[^a-z0-9-]/gi, "");
+      return serveGenerated(path.join(ROOT_DIR, `public/skills/${id}.html`));
+    },
+    "/anti-patterns": () => serveGenerated(path.join(ROOT_DIR, "public/anti-patterns/index.html")),
+    "/tutorials": () => serveGenerated(path.join(ROOT_DIR, "public/tutorials/index.html")),
+    "/tutorials/:slug": (req) => {
+      const slug = req.params.slug.replace(/[^a-z0-9-]/gi, "");
+      return serveGenerated(path.join(ROOT_DIR, `public/tutorials/${slug}.html`));
+    },
+
     // Static assets - all public subdirectories
     "/assets/*": async (req) => {
       const url = new URL(req.url);