Add nix flake to repo (#1111)

.github/CODEOWNERS

@@ -0,0 +1,3 @@
+# Nix stuff is owned by Rory& - we& want to be notified if these are changed.
+/flake.nix root@rory.gay
+/nix-update.sh root@rory.gay

.github/workflows/nix-build.yml

@@ -0,0 +1,18 @@
+name: Nix build
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build-nix:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: DeterminateSystems/magic-nix-cache-action@v2
+    - run: nix build -L
+    - run: nix develop --command echo OK

.gitignore

@@ -18,4 +18,5 @@ build
 *.log.ansi
 *.tmp
 tmp/
-dump/
+dump/
+result

.husky/pre-commit

@@ -1,4 +1,28 @@
 #!/usr/bin/env sh
+#!nix-shell -i "bash" -p bash prefetch-npm-deps jq nodejs nix-output-monitor
 . "$(dirname -- "$0")/_/husky.sh"
 
-npx -y lint-staged
+# Check if nix is available
+if [ -x "$(/usr/bin/env which nix-shell 2>/dev/null)" ]; then
+  # Check if we haven't re-executed ourselves yet
+  if [ ! "$HOOK_NIX_SHELL" ]; then
+    echo "Nix is available, updating nix flake..."
+    export HOOK_NIX_SHELL=1
+    nix-shell $0
+    exit $?
+  else
+    nix flake update
+    # run ./nix-update.sh if package lock has changed and has no unstaged changes
+    if [ -n "$(git status --porcelain=v1 2>/dev/null | grep -E '^(MM| M) package-lock.json')" ]; then
+      echo "package-lock.json has unstaged changes. Skipping update of nix dependencies."
+    elif [ ! -n "$(git status --porcelain=v1 2>/dev/null | grep -E '^M package-lock.json')" ]; then
+      echo "package-lock.json has no changes. Skipping update of nix dependencies."
+    else
+      ./nix-update.sh || exit $?
+    fi
+  fi
+else
+  echo "You do not appear to have nix installed. Skipping update of nix dependencies."
+fi
+
+npx -y lint-staged