Make Emacs flake standalone

flakes/emacs/README.md

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+# Tobmacs flake
+
+This flake builds the complete Emacs configuration without Home Manager. It
+contains Emacs, all Emacs packages, tree-sitter grammars, snippets, Hunspell,
+and dictionaries.
+
+## Run or install directly
+
+```console
+nix run 'git+https://codeberg.org/oibot/dotfiles?dir=flakes/emacs'
+nix profile install 'git+https://codeberg.org/oibot/dotfiles?dir=flakes/emacs'
+```
+
+From this repository, use `nix run path:./flakes/emacs`.
+
+## NixOS
+
+Add the flake as an input and make its nixpkgs follow the system nixpkgs:
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    tobs-emacs = {
+      url = "git+https://codeberg.org/oibot/dotfiles?dir=flakes/emacs";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, tobs-emacs, ... }: {
+    nixosConfigurations.server = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        tobs-emacs.nixosModules.default
+        {
+          # Installs Tobmacs and starts an Emacs user service. No Home Manager
+          # module is involved.
+          services.emacs.enable = true;
+          services.emacs.startWithGraphical = false;
+        }
+      ];
+    };
+  };
+}
+```
+
+If no daemon is wanted, either set `services.emacs.install = true` instead of
+`enable`, or install the package directly:
+
+```nix
+environment.systemPackages = [
+  tobs-emacs.packages.${pkgs.stdenv.hostPlatform.system}.default
+];
+```