wip: feat: Emacs config (from scratch }:3)

Working:
  - The daemon
  - Mutable config — well, kinda. I'm not yet sure how it will interact with
    `emacsWithPackagesFromUsePackage`.
  - Packages via Nixpkgs

users/crumb/programs/emacs.nix

@@ -0,0 +1,49 @@
+{ config, lib, pkgs, inputs, ... }@args:
+
+# NOTE: Remember to wrap `programs.emacs.finalPackage` rather than pass a
+# wrapped package to `programs.emacs.package`, lest some unexpected behaviour
+# occur when home-manager calls emacs during the build process.
+
+let
+  emacsPackage = pkgs.emacs30-pgtk;
+  emacsConfigDir = "/persist/dots/users/crumb/programs/emacs";
+
+  emacs-overlay =
+    import (builtins.fetchTarball {
+      url =
+        "https://github.com/nix-community/emacs-overlay/archive/master.tar.gz";
+      sha256 = "sha256:02aln37ch9isz8h7dlm9v6jkl60g923z0dij0rjsxq7xi61fas2j";
+    });
+
+  # Create a new instance of nixpkgs with emacs-overlay applied. This is a
+  # little unorthodox, but we do it
+  #   1. for the sake of organisation — For pure aesthetics and a clean codebase, I want
+  #      everything Emacs to stay in this file;
+  #   2. and, there's simply no need to leak emacs-overlay's packages into the
+  #      global nixpkgs instance when nothing else is using it!
+  pkgs' = import inputs.nixpkgs {
+    system = args.system;
+    overlays = [ emacs-overlay ];
+  };
+
+  my-emacs = pkgs'.emacsWithPackagesFromUsePackage {
+    config = ./emacs/init.el;
+    defaultInitFile = false;
+    package = emacsPackage;
+    alwaysEnsure = true;
+  };
+in {
+  programs.emacs = {
+    enable = true;
+    package = my-emacs;
+    extraConfig = ''
+      (load (file-name-concat "${emacsConfigDir}" "init"))
+    '';
+  };
+
+  services.emacs = {
+    enable = true;
+    # Generate a desktop entry for emacsclient.
+    client.enable = true;
+  };
+}