build: content-hash cache-busting for local CSS/JS (?v=hash)

Stops deploys being served stale from browser cache — the recurring "changes
do not show up until a hard refresh" problem.

- New tools/cache_bust.py stamps a global content hash onto every local
  <link>/<script> CSS/JS ref AND the @import lines in css/main.css (so partial
  edits bust too). External (https://) refs are untouched. Deterministic and
  idempotent: same content -> same stamp, any asset edit -> new stamp sitewide.
- Wired as the final step of tools/build_site.py so every build/deploy stamps.
- Initial stamp applied across 259 pages + main.css (v=64a53dd6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

404.html

@@ -25,7 +25,7 @@
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap" rel="stylesheet">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" crossorigin="anonymous" referrerpolicy="no-referrer">
-    <link rel="stylesheet" href="/css/main.css">
+    <link rel="stylesheet" href="/css/main.css?v=64a53dd6">
 
     <link rel="icon" type="image/png" href="/favicon-96x96.png" sizes="96x96">
     <link rel="icon" type="image/svg+xml" href="/favicon.svg">
@@ -70,7 +70,7 @@
             </p>
         </div>
     </main>
-<link rel="stylesheet" href="/assistant.css">
-<script src="/assistant.js" defer></script>
+<link rel="stylesheet" href="/assistant.css?v=64a53dd6">
+<script src="/assistant.js?v=64a53dd6" defer></script>
 </body>
 </html>