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/auth/already_logged_in.html

@@ -0,0 +1,12 @@
+{% extends "base.html" %}
+
+{% block title %}Already Logged In - Art-DAG{% endblock %}
+
+{% block content %}
+<div class="max-w-md mx-auto text-center">
+    <div class="bg-green-900/50 border border-green-700 text-green-300 px-4 py-3 rounded-lg mb-4">
+        You are already logged in as <strong>{{ user.username }}</strong>
+    </div>
+    <p><a href="/" class="text-blue-400 hover:text-blue-300">Go to home page</a></p>
+</div>
+{% endblock %}