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/Dockerfile

@@ -0,0 +1,23 @@
+FROM python:3.11-slim
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends git && rm -rf /var/lib/apt/lists/*
+
+# Install common as local package (no more git+https)
+COPY common/ /tmp/common/
+RUN pip install --no-cache-dir /tmp/common/ && rm -rf /tmp/common
+
+# Install L2 dependencies
+COPY l2/requirements.txt .
+ARG CACHEBUST=1
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy L2 application
+COPY l2/ .
+
+RUN mkdir -p /data/l2
+
+ENV PYTHONUNBUFFERED=1
+ENV ARTDAG_DATA=/data/l2
+
+CMD ["python", "server.py"]