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/requirements-dev.txt

@@ -0,0 +1,16 @@
+# Development dependencies
+-r requirements.txt
+
+# Type checking
+mypy>=1.8.0
+types-requests>=2.31.0
+types-PyYAML>=6.0.0
+typing_extensions>=4.9.0
+
+# Testing
+pytest>=8.0.0
+pytest-asyncio>=0.23.0
+pytest-cov>=4.1.0
+
+# Linting
+ruff>=0.2.0