First task of Phase 3b-i (notion semantic search). Adds _search_llm.py
that dispatches to the anthropic SDK when ANTHROPIC_API_KEY is set,
falls back to `claude -p` CLI otherwise, and raises a clear setup
error if neither works. Symlinks _notion_compat.py from 32-notion-writer
so both skills share one source of truth.
3 tests cover the three dispatch paths.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
