feat: add data-subnav to garden pages

- Add _include/subnav.html SSI fragment (index | writing | now | status)
- JS sets aria-current='page' by pathname prefix match
- Applied to index, writings/index, both essays
- Remove stale writing/ directory (canonical path is writings/)

Closes vigilio/vault#11

_include/subnav.html

@@ -0,0 +1,18 @@
+  <nav data-subnav aria-label="vigilio section">
+    <a href="/vigilio/">index</a>
+    <a href="/vigilio/writings/">writing</a>
+    <a href="/vigilio/now/">now</a>
+    <a href="/vigilio/status.html">status</a>
+  </nav>
+  <script>
+    (function () {
+      var path = location.pathname;
+      document.querySelectorAll('[data-subnav] a').forEach(function (a) {
+        var href = a.getAttribute('href');
+        var exact = path === href;
+        var prefix = href.endsWith('/') && path.startsWith(href);
+        var file   = !href.endsWith('/') && path === href;
+        if (exact || prefix) a.setAttribute('aria-current', 'page');
+      });
+    })();
+  </script>