Copy buttons on code blocks + merge Skills and Tutorials under Docs

Two small-to-medium improvements bundled together.

1. Copy buttons on every rendered code block.
   - render-markdown.js: wrap each fenced code block in a .code-block-wrap
     container and emit a <button class="code-block-copy" data-copy="...">
     alongside it. Button text is set via CSS ::before content so the
     'Copy' / 'Copied' label is a single toggle class (.is-copied).
   - render-page.js: 12-line inline script at the end of the body wires
     a delegated click handler that calls navigator.clipboard.writeText
     and flips .is-copied for 1.5s.
   - sub-pages.css: button styles matching the dark terminal palette,
     hidden until you hover the code block, accent-colored success state.

2. Merge Skills and Tutorials under a single Docs nav item.
   - Rename the Skills nav link to 'Docs' in every header (partial +
     4 hand-authored pages). Drop the separate Tutorials nav item; it
     now lives inside Docs. Anti-patterns stays as its own top-level.
   - scripts/build-sub-pages.js: replace renderSkillsSidebar and
     renderTutorialsSidebar with a unified renderDocsSidebar that shows
     every skill category followed by a Tutorials group. Takes a
     current descriptor of shape { kind: 'skill'|'tutorial', id|slug }
     so both skill detail and tutorial detail pages can mark the active
     row. activeNav on every /skills/* and /tutorials/* page is now
     'docs'; the shared site header's data-nav matches.

Verified: /skills/polish and /tutorials/getting-started both render
with the unified Docs sidebar (all 21 skills grouped by category +
both tutorials as a final group). The Docs nav item is aria-current
on both. Copy buttons appear on every fenced code block and toggle
to 'Copied' when clicked.

scripts/lib/render-page.js

@@ -102,6 +102,19 @@ export function renderPage({
   <main id="main">
 ${bodyHtml}
   </main>
+  <script>
+    // Copy buttons on rendered code blocks
+    document.addEventListener('click', (e) => {
+      const btn = e.target.closest('[data-copy]');
+      if (!btn) return;
+      const text = btn.getAttribute('data-copy');
+      if (!text) return;
+      navigator.clipboard.writeText(text).then(() => {
+        btn.classList.add('is-copied');
+        setTimeout(() => btn.classList.remove('is-copied'), 1500);
+      }).catch(() => {});
+    });
+  </script>
 </body>
 </html>
 `;