v1.3.0 - environment support + processing unit selection

New features:
- environment section in installer YAML config for passing env vars
  into containers (container_env mapping, CLI flags, .env defaults)
- built-in --processing-unit flag (cpu, cuda, rocm) in every generated
  installer - no opt-in required, any user can choose at start time
- --gpus flag for GPU device selection (maps to NVIDIA_VISIBLE_DEVICES
  or HIP_VISIBLE_DEVICES depending on processing unit)
- docker-compose.cuda.yml and docker-compose.rocm.yml overlays
  generated automatically, merged by compose() at runtime

Breaking changes:
- CLI flags renamed: -p -> --port, -c -> --cpus, -r -> --memory, -s -> --swap
- gpu: true YAML field removed (processing unit support is now unconditional)