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>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1776169885,
+        "narHash": "sha256-l/iNYDZ4bGOAFQY2q8y5OAfBBtrDAaPuRQqWaFHVRXM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "4bd9165a9165d7b5e33ae57f3eecbcb28fb231c9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}