fix(swarm): move docker-compose.swarm.yml back to root directory

- Move docker/docker-compose.swarm.yml to root
- Update documentation references
- Simplifies deployment: swarm file in root, standalone file in root
- Deploy script already expects file in root

Rationale: Keep it simple - two compose files for two deployment methods:
  - docker-compose.yml → standalone (docker compose up -d)
  - docker-compose.swarm.yml → swarm (docker stack deploy)

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

docs/SWARM-DEPLOYMENT.md

@@ -108,7 +108,7 @@ cd /opt/mosaic/stack
 
 # Or manually
 IMAGE_TAG=dev docker stack deploy \
-  -c docker/docker-compose.swarm.yml \
+  -c docker-compose.swarm.yml \
   --with-registry-auth mosaic
 ```