Incorporate art-dag-mono repo into artdag/ subfolder

Merges full history from art-dag/mono.git into the monorepo
under the artdag/ directory. Contains: core (DAG engine),
l1 (Celery rendering server), l2 (ActivityPub registry),
common (shared templates/middleware), client (CLI), test (e2e).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

git-subtree-dir: artdag
git-subtree-mainline: 1a179de547
git-subtree-split: 4c2e716558

artdag/l1/app/templates/fragments/nav_item.html

@@ -0,0 +1,7 @@
+<div class="relative nav-group">
+  <a href="{{ artdag_url }}"
+     class="justify-center cursor-pointer flex flex-row items-center gap-2 rounded bg-stone-200 text-black p-3"
+     data-hx-disable>
+    <i class="fas fa-project-diagram text-sm"></i> art-dag
+  </a>
+</div>