feat: add Nix flake for reproducible dev environment

flake.nix

@@ -0,0 +1,54 @@
+{
+  description = "LakeBench dev environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        python = pkgs.python313;
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = [
+            python
+            pkgs.uv
+            pkgs.ruff
+          ];
+
+          shellHook = ''
+            create-venv() {
+              rm -rf .venv
+              uv venv --python ${python}/bin/python .venv
+              source .venv/bin/activate
+
+              uv pip install -e .
+              # spark and sail conflict with each other, install separately
+              for extra in duckdb polars daft tpcds_datagen tpch_datagen sparkmeasure spark sail; do
+                uv pip install "lakebench[$extra]" 2>&1 || echo "warning: $extra failed to install, skipping"
+              done
+              uv pip install --group dev
+              uv pip install jupyter ipykernel
+              python -m ipykernel install --user --name python3 --display-name "Python 3 (LakeBench)"
+
+              # store hash to detect pyproject.toml changes
+              md5sum pyproject.toml > .venv/.pyproject.hash
+            }
+
+            if [ ! -d .venv ]; then
+              create-venv
+            elif ! md5sum --check .venv/.pyproject.hash &>/dev/null; then
+              echo "pyproject.toml changed, recreating venv..."
+              create-venv
+            else
+              source .venv/bin/activate
+            fi
+          '';
+        };
+      }
+    );
+}