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/.env.example

@@ -0,0 +1,20 @@
+# L2 Server Configuration
+
+# PostgreSQL password (REQUIRED - no default)
+POSTGRES_PASSWORD=changeme-generate-with-openssl-rand-hex-16
+
+# Domain for this ActivityPub server
+ARTDAG_DOMAIN=artdag.rose-ash.com
+
+# JWT secret for token signing (generate with: openssl rand -hex 32)
+JWT_SECRET=your-secret-here-generate-with-openssl-rand-hex-32
+
+# L1 server URL for fetching content (images/videos)
+L1_PUBLIC_URL=https://celery-artdag.rose-ash.com
+
+# Effects repository URL for linking to effect source code
+EFFECTS_REPO_URL=https://git.rose-ash.com/art-dag/effects
+
+# Notes:
+# - ARTDAG_USER removed - now multi-actor, each registered user is their own actor
+# - L1 URL can also come from provenance data per-asset