Add sub-page render pipeline foundation

Groundwork for new /skills, /anti-patterns, /tutorials sections.
No user-visible changes yet — this is pure plumbing.

- Split main.css into tokens.css (design tokens + reset, ~100 lines) and
  main.css (everything else, imports tokens.css). Lets sub-pages import
  only tokens without pulling in the landing-page component CSS.
- Add marked as a dependency.
- Add scripts/lib/render-markdown.js: marked wrapper with a custom link
  resolver (skill slugs, reference/*.md anchors, external rel=noopener),
  stable heading slugger, and terminal-style code blocks.
- Add scripts/lib/render-page.js: page shell wrapper that injects the
  shared site header partial with aria-current marking.
- Add content/site/partials/header.html: shared site header with nav
  (Home / Skills / Anti-Patterns / Tutorials / Gallery / GitHub).
- Add public/css/sub-pages.css: shared styles for generated pages, with
  .site-header styling (sticky, backdrop blur, accent-underlined active
  nav item) and mobile collapse.

Build still produces the same 104 KB landing-page CSS chunk; tests pass.

bun.lock

@@ -6,6 +6,7 @@
       "name": "vibe-design-plugins",
       "dependencies": {
         "jsdom": "^29.0.0",
+        "marked": "^16.1.0",
       },
       "devDependencies": {
         "archiver": "^7.0.1",
@@ -381,6 +382,8 @@
 
     "lru-cache": ["lru-cache@11.2.7", "", {}, "sha512-aY/R+aEsRelme17KGQa/1ZSIpLpNYYrhcrepKTZgE+W3WM16YMCaPwOHLHsmopZHELU0Ojin1lPVxKR0MihncA=="],
 
+    "marked": ["marked@16.4.2", "", { "bin": { "marked": "bin/marked.js" } }, "sha512-TI3V8YYWvkVf3KJe1dRkpnjs68JUPyEa5vjKrp1XEEJUAOaQc+Qj+L1qWbPd0SJuAdQkFU0h73sXXqwDYxsiDA=="],
+
     "mdn-data": ["mdn-data@2.27.1", "", {}, "sha512-9Yubnt3e8A0OKwxYSXyhLymGW4sCufcLG6VdiDdUGVkPhpqLxlvP5vl1983gQjJl3tqbrM731mjaZaP68AgosQ=="],
 
     "miniflare": ["miniflare@4.20260317.0", "", { "dependencies": { "@cspotcode/source-map-support": "0.8.1", "sharp": "^0.34.5", "undici": "7.24.4", "workerd": "1.20260317.1", "ws": "8.18.0", "youch": "4.1.0-beta.10" }, "bin": { "miniflare": "bootstrap.js" } }, "sha512-xuwk5Kjv+shi5iUBAdCrRl9IaWSGnTU8WuTQzsUS2GlSDIMCJuu8DiF/d9ExjMXYiQG5ml+k9SVKnMj8cRkq0w=="],