Refactor Output Generators

[baync180705]

Move the output package logic to internal/core/generator. These functions are responsible for creating the artifacts (files) that go into the backup.

• Create directory internal/core/generator.
• Move output/json.go → internal/core/generator/metadata.go.
  -- Rename function to GenerateMetadata(...).
• Move output/script.go → internal/core/generator/scripts.go.
  -- Ensure it uses the new domain types if necessary.
• Tarball Logic:
  -- The tarball.go file currently orchestrates the writing. This logic should eventually move to internal/core/orchestrator.go or a dedicated internal/platform/archive helper. For now, move it to internal/core/generator/archive.go.