add Vale for prose linting

formatter.nix

@@ -45,5 +45,55 @@
         ];
         includes = [ "*.py" ];
       };
+      treefmt.settings.formatter.vale =
+        with pkgs;
+        let
+          vocab = "cLAN";
+          style = "Docs";
+          config = writeText "vale.ini" ''
+            StylesPath = ${styles}
+            Vocab = ${vocab}
+
+            [*.md]
+            BasedOnStyles = Vale, ${style}
+            Vale.Terms = No
+          '';
+          styles = symlinkJoin {
+            name = "vale-style";
+            paths = [
+              accept
+              headings
+            ];
+          };
+          accept = writeTextDir "config/vocabularies/${vocab}/accept.txt" ''
+            cLAN
+            Nix
+            NixOS
+            Nixpkgs
+            clan.lol
+            Clan
+            monorepo
+          '';
+          headings = writeTextDir "${style}/headings.yml" ''
+            extends: capitalization
+            message: "'%s' should be in sentence case"
+            level: error
+            scope: heading
+            # $title, $sentence, $lower, $upper, or a pattern.
+            match: $sentence
+          '';
+        in
+        {
+          command = "${vale}/bin/vale";
+          options = [ "--config=${config}" ];
+          includes = [ "*.md" ];
+          # TODO: too much at once, fix piecemeal
+          excludes = [
+            "README.md"
+            "docs/*"
+            "clanModules/*"
+            "pkgs/*"
+          ];
+        };
     };
 }