fix: escape dollar signs in Dokploy project-level variables

Docker Compose interprets $ as variable substitution, so we need to escape
Dokploy's project-level variable syntax by doubling the dollar sign.

Changes:
- docker-compose.*.yml: ${{project.VAR}} → $${{project.VAR}}
- Updated DOKPLOY_DEPLOYMENT.md with correct syntax and explanation
- Updated SHARED_PROJECT_DEPLOYMENT.md with correct syntax and explanation

This fixes the 'You may need to escape any $ with another $' error when
deploying via Dokploy.

Evidence: Tested in Dokploy deployment - error resolved with $$ escaping.

docker-compose.staging.yml

@@ -15,8 +15,8 @@ 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=$${{project.SHARED_PROJECT_ID}}
+      - SHARED_ENVIRONMENT_ID=$${{project.SHARED_ENVIRONMENT_ID}}
     env_file:
       - .env
     restart: unless-stopped