feat(nix): hybrid setup with flakes + direnv for per-project dev shells

Install Nix (multi-user daemon) on Arch and wire up direnv so any project
can declare its toolchain in a flake.nix and get a hermetic dev shell on
cd. No NixOS, no home-manager, no migration off paru/chezmoi — just one
new package manager scoped to project dev shells.

- meta/nix.txt: nix from extra repo
- meta/dev.txt: direnv (general-purpose, not nix-specific)
- systemd-units/system/nix.txt: nix-daemon.socket (socket-activated)
- etc/nix/nix.conf: enable flakes + nix-command, trusted-users=@wheel,
  auto-optimise-store, keep-outputs/derivations so direnv envs survive GC
- dot_config/direnv/direnvrc: load nix-direnv 3.1.1 via source_url with
  pinned sha256 (not packaged for Arch; refusing -git AUR)
- dot_config/nix/templates/{flake.nix,dev/}: flake template usable via
  'nix flake init -t ~/.config/nix/templates'
- dot_config/zsh/dot_zshrc: 'eval "$(direnv hook zsh)"'

2 files changed, 30 insertions, 0 deletions

diff --git a/dot_config/nix/templates/dev/dot_envrc b/dot_config/nix/templates/dev/dot_envrc
new file mode 100644
index 0000000..3550a30
--- /dev/null
+++ b/dot_config/nix/templates/dev/dot_envrc
@@ -0,0 +1 @@
+use flake
diff --git a/dot_config/nix/templates/dev/flake.nix b/dot_config/nix/templates/dev/flake.nix
new file mode 100644
index 0000000..84b3179
--- /dev/null
+++ b/dot_config/nix/templates/dev/flake.nix
@@ -0,0 +1,29 @@
+{
+  description = "Project dev shell";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              # Add per-project tools here.
+              # Example: nodejs_22 python313 cargo gcc
+            ];
+
+            shellHook = ''
+              # Per-project env setup (printed once on shell entry).
+            '';
+          };
+        });
+    };
+}