init: LightRAG + Graphiti as submodules with Nix flake

Impure devShells using uv + Python 3.12 for each project.
LD_LIBRARY_PATH set for NixOS native wheel compatibility.
Neo4j managed via nix run .#neo4j-start / neo4j-stop.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.graphiti

@@ -0,0 +1,14 @@
+# Neo4j (started via: nix run .#neo4j-start)
+NEO4J_URI=bolt://localhost:7687
+NEO4J_USER=neo4j
+NEO4J_PASSWORD=neo4j
+
+# LLM via Ollama (OpenAI-compatible)
+OPENAI_API_KEY=ollama
+OPENAI_BASE_URL=http://localhost:11434/v1
+MODEL_NAME=qwen2.5:7b
+
+# Embeddings via Ollama
+# nomic-embed-text dim=768, mxbai-embed-large dim=1024
+EMBEDDING_MODEL=nomic-embed-text
+EMBEDDING_DIM=768