[UltiPro]

Summary

Adds the core domain model and the first ports, as the base for the metadata schema.

• core/versions.py: SemVer with parsing, ordering and bumping
• core/entities.py: Model, ModelVersion, Run and Alias as immutable dataclasses, with the rules for releasing, renumbering and deleting a version
• core/errors.py: the errors that miniOPs reports to the user
• ports/: MetadataStore and ArtifactStore in detail; Executor, ServingBackend and ImageRegistry are provisional
• README.md: the docker target uses the same state as local

Checklist

• PR title follows Conventional Commits (e.g. feat(registry): ...)
• Larger or architectural changes were discussed in an issue first (n/a: maintainer's design work from the roadmap)
• Tests added or updated (if applicable)
• No OS-specific assumptions (paths, shell commands, file encodings); works on Linux, macOS and Windows
• Documentation updated
• CHANGELOG.md updated under "Unreleased"