fix(demo): wire every showcase path end-to-end with auto-resolved artifacts

chore(rag): gitignore external clinical RAG corpus and pre-built index

fix(gitignore): blanket-ignore data/knowledge_base/ except README+.gitkeep

feat(rag): add fastembed/faiss-cpu/pypdf for retrieval layer

chore: add real-llm-rationale plan + ignore .worktrees/

feat(llm): OpenRouter free-tier fallback chain (smartest → smallest)

chore: ignore all .venv*/ and venv*/ directories

chore: track planning docs and ignore .sixth/ tooling dir

chore: bootstrap NeuroBridge project skeleton