Simplify CI: remove GPU workflow, use registry images

- Remove gpu-worker.yml (no GPU server)
- Fix ci.yml: install ssh/rsync in job container, remove GPU steps
- Remove source mounts from l1-server and l1-worker so they use image code

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -82,8 +82,6 @@ services:
       # L2_SERVER, L2_DOMAIN, IPFS_GATEWAY_URL from .env file
     volumes:
       - l1_cache:/data/cache
-      # Mount source code for development - restart service to pick up changes
-      - .:/app
     depends_on:
       - redis
       - postgres
@@ -113,8 +111,6 @@ services:
       - ARTDAG_CLUSTER_KEY=${ARTDAG_CLUSTER_KEY:-}
     volumes:
       - l1_cache:/data/cache
-      # Mount source code for development - restart service to pick up changes
-      - .:/app
     depends_on:
       - redis
       - postgres