feat: support compose-style interpolation in x-topo attributes

[awphi]

Changes

• Adds support for compose-style env var interpolation inside x-topo.deployment_success_message
• Injects two new env vars when running compose.ReadProject in the post deploy step:
  • TOPO_TARGET: the selected target for deploy/stop/ps
  • TOPO_TARGET_HOSTNAME: the ssh -G-resolved HostName of the selected target

name: test-project
services:
  app:
    image: nginx
x-topo:
  deployment_success_message: "${COMPOSE_PROJECT_NAME} is available at http://${TOPO_TARGET_HOSTNAME}:${APP_PORT:-8080}"

Checklist

• 🤖 This change is covered by tests as required.
• 🤹 All required manual testing has been performed.
• 📖 All documentation updates are complete.

[muchzill4]

Wonder if we should scope it to success_message for now. It’s nice to keep the behaviour consistent, but using TOPO_TARGET_HOSTNAME anywhere else sounds a bit nuts.

[muchzill4]

docker compose doesn’t read x-topo anyway, so no need for a default.