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.lightrag

@@ -0,0 +1,18 @@
+# LLM via Ollama
+LLM_BINDING=ollama
+LLM_MODEL=qwen2.5:7b
+LLM_BINDING_HOST=http://localhost:11434
+
+# Embeddings via Ollama
+EMBEDDING_BINDING=ollama
+EMBEDDING_MODEL=nomic-embed-text
+EMBEDDING_BINDING_HOST=http://localhost:11434
+EMBEDDING_DIM=768
+
+# Storage (local files)
+RAG_DIR=./rag_storage
+
+# Server
+HOST=0.0.0.0
+PORT=9621
+WORKERS=1