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>

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "lightrag"]
+	path = lightrag
+	url = https://github.com/hkuds/lightrag
+[submodule "graphiti"]
+	path = graphiti
+	url = https://github.com/getzep/graphiti