fix: Use standard Docker Compose variable syntax

Changed from ${{project.VAR}} to ${VAR} syntax.

The ${{project.VAR}} syntax is for Dokploy's Environment editor UI,
not for docker-compose.yml files. Docker Compose requires standard
${VAR} syntax to read from .env file.

The .env file (managed by Dokploy) already contains the actual values.

docker-compose.staging.yml

@@ -13,8 +13,8 @@ services:
       - STACK_DOMAIN_SUFFIX=${STACK_DOMAIN_SUFFIX:-ai.flexinit.nl}
       - STACK_IMAGE=${STACK_IMAGE:-git.app.flexinit.nl/flexinit/agent-stack:latest}
       - RESERVED_NAMES=${RESERVED_NAMES:-admin,api,www,root,system,test,demo,portal}
-      - SHARED_PROJECT_ID=${{project.SHARED_PROJECT_ID}}
-      - SHARED_ENVIRONMENT_ID=${{project.SHARED_ENVIRONMENT_ID}}
+      - SHARED_PROJECT_ID=${SHARED_PROJECT_ID}
+      - SHARED_ENVIRONMENT_ID=${SHARED_ENVIRONMENT_ID}
     env_file:
       - .env
     restart: unless-stopped