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/app/repositories/__init__.py

@@ -0,0 +1,10 @@
+"""
+L1 Server Repositories.
+
+Data access layer for persistence operations.
+"""
+
+# TODO: Implement repositories
+# - RunRepository - Redis-backed run storage
+# - RecipeRepository - Redis-backed recipe storage
+# - CacheRepository - Filesystem + PostgreSQL cache metadata