flake: enable treefmt for formatting

- for now, uses only `nixfmt-rfc-style`
- can be invoked with `nix flake check` and `nix fmt`

flake.nix

@@ -14,6 +14,10 @@
       # deliberately do _not_ follow the nixpkgs input here, because paranoia and test coverage
     };
     utils.url = "github:numtide/flake-utils";
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
 
       nur.url = "github:nix-community/NUR";
     # TODO: possible make this a flake as well
@@ -25,7 +29,7 @@
   };
 
   outputs = 
-  { self, nixpkgs, nur, lanzaboote, utils, home-manager, ... }@inputs: 
+  { self, nixpkgs, nur, lanzaboote, utils, home-manager, treefmt-nix, ... }@inputs:
   let
     # FIXME: allow different systems
     systems = utils.lib.system;
@@ -71,6 +75,10 @@
         inherit inputs system;
       };
     };
+    treefmtConf = {pkgs, ...}:{
+      projectRootFile = "flake.nix";
+      programs.nixfmt-rfc-style.enable = true;
+    };
   in
   {
     overlays.default = import ./packages;
@@ -85,11 +93,19 @@
         inherit system;
         overlays = nixpkgs.lib.attrValues self.overlays;
       };
+      nixpkgs_unstable = import inputs.nixos-unstable {
+        inherit system;
+        overlays = nixpkgs.lib.attrValues self.overlays;
+      };
       pkgs = inputs.nixpkgs.legacyPackages.${system};
+      pkgs_unstable = inputs.nixos-unstable.legacyPackages.${system};
+      treefmtEval = treefmt-nix.lib.evalModule pkgs_unstable treefmtConf;
     in {
       devShells.default = pkgs.mkShell {
         buildInputs = [];
       };
+      formatter = treefmtEval.config.build.wrapper;
+      checks.formatting = treefmtEval.config.build.check self;
       # expose nixpkgs with overlay; TODO: figure out role of config
       #legacyPackages = nixpkgs;
     });