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/.gitignore

@@ -0,0 +1,11 @@
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+
+# Private keys - NEVER commit these
+*.pem
+keys/
+
+# Secrets
+.env