♻️ infra: flake.nix + nix-based CI

flake.nix: gleam dev shell via erlang_27 + pkgs.gleam + rebar3.
Matches ~/.flakes#gleam. flake.lock committed — pinned to nixos-unstable
2026-02-27.

CI: nix develop -c gleam test + format --check. Uses
systemic-engineering/ci/actions/nix-setup for Nix install + magic cache.
Local/CI parity: same commands, same environment.

Co-Authored-By: Mara <mara@systemic.engineer>

Author: Mara

.github/workflows/test.yml

@@ -1,23 +1,23 @@
-name: test
+name: CI
 
 on:
   push:
-    branches:
-      - master
-      - main
+    branches: [main]
   pull_request:
 
 jobs:
   test:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: erlef/setup-beam@v1
-        with:
-          otp-version: "28"
-          gleam-version: "1.14.0"
-          rebar3-version: "3"
-          # elixir-version: "1"
-      - run: gleam deps download
-      - run: gleam test
-      - run: gleam format --check src test
+
+      - uses: systemic-engineering/ci/actions/nix-setup@main
+
+      - name: Download deps
+        run: nix develop -c gleam deps download
+
+      - name: Test
+        run: nix develop -c gleam test
+
+      - name: Format check
+        run: nix develop -c gleam format --check src test

flake.lock

@@ -0,0 +1,25 @@
+{
+  description = "fragmentation — content-addressed, arbitrary-depth fragment trees";
+
+  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     = nixpkgs.legacyPackages.${system};
+        beamPkgs = pkgs.beam.packages.erlang_27;
+        erlang   = pkgs.erlang_27;
+        gleam    = pkgs.gleam;
+        rebar3   = beamPkgs.rebar3;
+      in {
+        devShells.default = pkgs.mkShell {
+          buildInputs = [ gleam erlang rebar3 pkgs.git pkgs.just ];
+          shellHook = ''
+            export LANG=en_US.UTF-8
+          '';
+        };
+      });
+}