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/deploy.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+set -e
+
+cd "$(dirname "$0")"
+
+echo "=== Pulling latest code ==="
+git pull
+
+echo "=== Building Docker image ==="
+docker build --build-arg CACHEBUST=$(date +%s) -t git.rose-ash.com/art-dag/l2-server:latest .
+
+echo "=== Redeploying activitypub stack ==="
+docker stack deploy -c docker-compose.yml activitypub
+
+echo "=== Restarting proxy nginx ==="
+docker service update --force proxy_nginx
+
+echo "=== Done ==="
+docker stack services activitypub