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/core/.gitignore

@@ -0,0 +1,47 @@
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# Testing
+.pytest_cache/
+.coverage
+htmlcov/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# Environment
+.env
+.venv
+env/
+venv/
+
+# Private keys (ActivityPub secrets)
+.cache/
+
+# Test outputs
+test_cache/
+test_plan_output.json
+analysis.json
+plan.json
+plan_with_analysis.json