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/check_redis.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python3
+"""Check Redis connectivity."""
+
+import redis
+
+try:
+    r = redis.Redis(host='localhost', port=6379, db=0)
+    r.ping()
+    print("Redis: OK")
+except redis.ConnectionError:
+    print("Redis: Not running")
+    print("Start with: sudo systemctl start redis-server")