KURE embedding pilot shadow index implemented

Result

• Branch: wip/kure-embedding-pilot
• Docs commit: 4946665 docs(context): preserve vector DB embedding research decision
• Code commit: d13c556 feat(vectorize): add KURE embedding pilot shadow index
• Added KURE provider config defaults: nlpai-lab/KURE-v1, 1024, profile kure.
• Added Neo4j shadow property/index support: embedding_kure_1024, maester_chunk_embedding_kure_1024.
• Kept KURE metadata separate as embedding_kure_model, embedding_kure_dim, etc.
• Added backfill CLI overrides for provider/model/dim/endpoint.

Verification

• py_compile passed for vector/graph/scripts/tools files.
• scripts/test_vectorize.py passed.
• KURE dry-run backfill passed for 3 docs.
• Neo4j query/index probe passed with nonzero vector.

Remaining

Actual KURE vector generation needs either MAESTER_KURE_ENDPOINT pointing to an OpenAI-compatible embedding server, or local optional sentence-transformers/torch installation. Current maester venv does not yet include sentence-transformers/torch.