feat: add Nix flake dev shell

CONTRIBUTING.md

@@ -13,6 +13,8 @@ npm test         # runs the Node, Python, and Go suites
 
 Prerequisites: Node 24+, a Python >=3.10 (the Python SDK pins 3.12 via `packages/sdk-py/.python-version`), and Go 1.26.6+. The `setup:*` scripts detect each toolchain and print install hints if it is missing. Both are idempotent and machine-local (the Python `.venv` is git-ignored), so re-run them after cloning or switching machines.
 
+> **Nix shortcut:** with [Nix](https://nixos.org/download/) installed, `nix develop` (from the repo root, see [`flake.nix`](flake.nix)) drops you into a shell with Node 24, Python 3.12, Go 1.26.6, `goreleaser`, `git`, and `jq` pre-installed — no manual toolchain setup needed. Note that `nix develop` always starts `bash`; to keep your own shell, run `nix develop -c fish` (or `zsh`, etc. — any shell already installed on your system). Direnv users can `echo "use flake" > .envrc && direnv allow` instead; direnv keeps your current shell.
+
 **Running your work-in-progress CLI**, two channels, one machine-wide at a time:
 
 - `npm run cli:live` links the source tree; rebuilds flow through instantly. For iterating.

flake.nix

@@ -0,0 +1,69 @@
+{
+  description = "Leji dev environment — Node 24, Python 3.12, Go 1.26.6";
+
+  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; };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            nodejs_24
+            python312
+            python312Packages.pip
+            go
+            goreleaser
+            git
+            jq
+          ];
+
+          shellHook = ''
+            if [ -z "$NO_COLOR" ] && [ -t 1 ]; then
+              _R=$(printf '\033[0m')
+              _B=$(printf '\033[38;2;0;159;113m')
+              _D=$(printf '\033[38;2;24;61;59m')
+              _M=$(printf '\033[38;2;112;216;194m')
+              _DIM=$(printf '\033[2m')
+              _BD=$(printf '\033[1m')
+            else
+              _R=""; _B=""; _D=""; _M=""; _DIM=""; _BD=""
+            fi
+            _node=$(node --version 2>/dev/null || echo "—")
+            _py=$(python3 --version 2>&1 | awk '{print $2}'); [ -z "$_py" ] && _py="—"
+            _pip=$(pip --version 2>/dev/null | awk '{print $2}'); [ -z "$_pip" ] && _pip="—"
+            _go=$(go version 2>/dev/null | awk '{print $3}' | sed 's/go//'); [ -z "$_go" ] && _go="—"
+            _gr=$(goreleaser --version 2>/dev/null | head -n1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+' | head -n1); [ -z "$_gr" ] && _gr="—"
+            _git=$(git --version 2>/dev/null | awk '{print $3}'); [ -z "$_git" ] && _git="—"
+            _jq=$(jq --version 2>/dev/null | sed 's/jq-//'); [ -z "$_jq" ] && _jq="—"
+            printf "\n"
+            printf "  %s%sleji%s %s•%s %sdevelopment shell%s\n" "$_B" "$_BD" "$_R" "$_M" "$_R" "$_DIM" "$_R"
+            printf "\n"
+            printf "  %s─ packages ─────────────────────────────────────%s\n" "$_DIM" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s)%s\n" "$_M" "$_R" "node" "$_BD" "$_node" "$_R" "$_DIM" "nodejs_24" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s, pip %s)%s\n" "$_M" "$_R" "python" "$_BD" "$_py" "$_R" "$_DIM" "python312" "$_pip" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s)%s\n" "$_M" "$_R" "go" "$_BD" "$_go" "$_R" "$_DIM" "go 1.26.6" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s)%s\n" "$_M" "$_R" "goreleaser" "$_BD" "$_gr" "$_R" "$_DIM" "goreleaser" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s)%s\n" "$_M" "$_R" "git" "$_BD" "$_git" "$_R" "$_DIM" "git" "$_R"
+            printf "    %s◆%s %-12s %s%s%s  %s(%s)%s\n" "$_M" "$_R" "jq" "$_BD" "$_jq" "$_R" "$_DIM" "jq" "$_R"
+            printf "  %s────────────────────────────────────────────────%s\n" "$_DIM" "$_R"
+            printf "\n"
+            unset _R _B _D _M _DIM _BD _node _py _pip _go _gr _git _jq
+          '';
+
+          env.PYTHON = "${pkgs.python312}/bin/python3";
+        };
+      }
+    );
+}