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/l2/app/templates/404.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+
+{% block title %}Not Found - Art-DAG{% endblock %}
+
+{% block content %}
+<div class="text-center py-16">
+    <h2 class="text-6xl font-bold text-gray-600 mb-4">404</h2>
+    <p class="text-xl text-gray-400 mb-8">Page not found</p>
+    <a href="/" class="text-blue-400 hover:text-blue-300">Go to home page</a>
+</div>
+{% endblock %}