Add treefmt formatter (nix fmt + flake check gate)

flake.lock

@@ -166,7 +166,8 @@
         "nixpkgs": [
           "clan-core",
           "nixpkgs"
-        ]
+        ],
+        "treefmt-nix": "treefmt-nix_2"
       }
     },
     "sops-nix": {
@@ -225,6 +226,26 @@
         "repo": "treefmt-nix",
         "type": "github"
       }
+    },
+    "treefmt-nix_2": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1780220602,
+        "narHash": "sha256-eynAfOmbmxJnkp7YewvCEbShNnnYJ9gLLqkzsYtBPeM=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "db947814a175b7ca6ded66e21383d938df01c227",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -1,6 +1,8 @@
 {
   inputs.clan-core.url = "https://git.clan.lol/clan/clan-core/archive/25.11.tar.gz";
   inputs.nixpkgs.follows = "clan-core/nixpkgs";
+  inputs.treefmt-nix.url = "github:numtide/treefmt-nix";
+  inputs.treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
 
   outputs =
     {
@@ -31,24 +33,36 @@
         "age-plugin-fido2-hmac"
       ];
       };
+
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "aarch64-darwin"
+        "x86_64-darwin"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+      pkgsFor = system: clan-core.inputs.nixpkgs.legacyPackages.${system};
+      treefmtFor = system: inputs.treefmt-nix.lib.evalModule (pkgsFor system) ./fmt.nix;
     in
     {
       inherit (clan.config) nixosConfigurations nixosModules clanInternals;
       clan = clan.config;
+
+      # `nix fmt` and the `nix flake check` formatting gate.
+      formatter = forAllSystems (system: (treefmtFor system).config.build.wrapper);
+      checks = forAllSystems (system: {
+        formatting = (treefmtFor system).config.build.check self;
+      });
+
       # Add the Clan cli tool to the dev shell.
       # Use "nix develop" to enter the dev shell.
-      devShells =
+      devShells = forAllSystems (system: {
-        nixpkgs.lib.genAttrs
+        default = (pkgsFor system).mkShell {
-          [
+          packages = [
-            "x86_64-linux"
+            clan-core.packages.${system}.clan-cli
-            "aarch64-linux"
+            (treefmtFor system).config.build.wrapper
-            "aarch64-darwin"
+          ];
-            "x86_64-darwin"
+        };
-          ]
+      });
-          (system: {
-            default = clan-core.inputs.nixpkgs.legacyPackages.${system}.mkShell {
-              packages = [ clan-core.packages.${system}.clan-cli ];
-            };
-          });
     };
 }

fmt.nix

@@ -0,0 +1,32 @@
+# treefmt config, evaluated per-system in flake.nix and exposed as
+# `nix fmt` (the formatter) plus a `nix flake check` formatting gate.
+{ ... }:
+{
+  projectRootFile = "flake.nix";
+
+  programs = {
+    nixfmt.enable = true;
+    prettier.enable = true;
+    yamlfmt.enable = true;
+    shfmt.enable = true;
+  };
+
+  settings = {
+    on-unmatched = "fatal";
+    global.excludes = [
+      # Secrets and clan-managed state — never reformat.
+      "sops/*"
+      "vars/*"
+      "inventory.json"
+
+      # Generated — don't reformat (regeneration would churn the diff).
+      "*facter.json"
+
+      # No formatter, or reformatting would corrupt them.
+      "*.zone" # Knot zone files
+      "flake.lock"
+      ".envrc"
+      ".gitignore"
+    ];
+  };
+}