chore: use ghcr.io image for production deployment

- Update prod docker-compose to pull from ghcr.io/xe138/grist-mcp-server
- Remove debug step from Gitea workflow

deploy/prod/docker-compose.yml

@@ -1,9 +1,7 @@
 # Production environment - resource limits, logging, restart policy
 services:
   grist-mcp:
-    build:
-      context: ../..
-      dockerfile: Dockerfile
+    image: ghcr.io/xe138/grist-mcp-server:latest
     ports:
       - "${PORT:-3000}:3000"
     volumes: