Version 0.1.8 release

Release 0.1.8 fixes selected-kind LLM generation for directory inputs and makes
product generation explicit.

Highlights

• open-data-products generate --kind=product-reference --input <source_docs/>
  now processes each .md and .txt source document separately with the ODPC
  product reference prompt.
• open-data-products generate --kind=odps-product --input <source_docs/>
  generates full ODPS OpenDataProduct YAML documents.
• Folder input now runs one selected-kind generation attempt per source file, so
  a folder of product source documents can produce matching
  product_reference_*.yaml fragments or odps_product_*.yaml specs in the
  output folder.
• open-data-products generate now requires an explicit concrete --kind
  value; the old full-set generation mode and ambiguous --kind=product value
  have been removed from the CLI.
• The public Python API now exposes generate_local_artifacts_for_kind() for
  selected-kind multi-source generation.