Use LinkML-Store for embedding generation

[caufieldjh]

Summary

• replace the active CurateGPT embedding backend with LinkML-Store's LLM indexer
• export LinkML-Store indexed vectors as a stable embeddings CSV column and load them from CSV/DuckDB
• remove the CurateGPT dependency and add LinkML-Store/llm/tiktoken dependencies
• document custom embedding models and OpenAI-compatible endpoint plugin setup

Validation

• poetry run pytest -q -> 59 passed, 3 skipped, 1 upstream LinkML-Store/Pydantic deprecation warning
• RUN_OPENAI_INTEGRATION=1 poetry run pytest tests/integration/test_generate_embeddings_integration.py -q -> 1 passed, 1 upstream LinkML-Store/Pydantic deprecation warning
• poetry check -> passed with Poetry metadata deprecation warnings
• poetry run flake8 --ignore E501,W503 src/trowel/utils/embedding_generation_utils.py src/trowel/utils/embedding_utils.py src/trowel/cli.py tests/test_embedding_generation.py tests/test_embedding_utils.py tests/test_cli_embeddings.py

Closes #20
Refs #14