- Add ComposeManager for handling base image resolution and compose operations
- Support multiple base image formats: ubuntu:24.04, debian/12/x86_64, etc.
- Implement compose subcommands: create, build-image, list
- Add dry-run support for safe testing without OSTree environment
- Map base images to OSTree branches: ubuntu:24.04 -> ubuntu/24.04/x86_64
- Support package specification and output branch control
- Temporarily disable OSTree validation for compose commands to enable testing
This enables the critical path for dogfooding with apt-ostree compose create --base ubuntu:24.04
