nix/docs: demote index sections h2 → h3 (argus #622 nit)

argus on #622 comment 7419 🟡:
> monospace h2 on index — the CSS styles `h2` with `ui-monospace`
> font (intended for option-name headings in the per-option pages).
> on the index page, "host options" / "agent options" / "Regenerate"
> headings also get monospace treatment. cosmetic; reads as
> intentional if not, easy to scope.

Index page now uses h3 for section headers, leaves h2 free for the
auto-generated option-name headings on host.html / agent.html where
the monospace styling is appropriate.

(Other argus nit — stale namespace text in indexHTML's option
listings — will be addressed when this branch rebases post-#615
merge, same as #620.)

nix/docs.nix

@@ -286,11 +286,11 @@ let
       echo '<main>'
       echo '<h1>hyperhive — nix options reference</h1>'
       echo '<p>Auto-generated from the <a href="${forgeRoot}">hyperhive flake</a>. Two reading paths:</p>'
-      echo '<h2><a href="./host.html">host options</a></h2>'
+      echo '<h3><a href="./host.html">host options</a></h3>'
       echo '<p>Options exposed by <code>hyperhive.nixosModules.default</code> to operator host configurations: <code>services.hive-c0re.*</code>, <code>hyperhive.domain</code>, <code>hyperhive.forge.*</code>, <code>hyperhive.matrix.*</code>.</p>'
-      echo '<h2><a href="./agent.html">agent options</a></h2>'
+      echo '<h3><a href="./agent.html">agent options</a></h3>'
       echo '<p>Per-agent options declared in <code>nix/templates/harness-base.nix</code> and visible from every <code>agent.nix</code>: <code>hyperhive.model</code>, <code>hyperhive.allowedRecipients</code>, <code>hyperhive.extraMcpServers</code>, <code>hyperhive.frontend.*</code>, <code>hyperhive.forge.*</code>, <code>hyperhive.matrix.*</code>, <code>hyperhive.gui.*</code>.</p>'
-      echo '<h2>Regenerate</h2>'
+      echo '<h3>Regenerate</h3>'
       echo '<pre><code>nix build .#docs          # bundled static site (index + host + agent)'
       echo 'nix build .#docs-host     # host page only (HTML)'
       echo 'nix build .#docs-agent    # agent page only (HTML)</code></pre>'