refactor(shell): use default modules for grouped tools

modules/shell/tmux/default.nix

@@ -0,0 +1,33 @@
+{
+  config,
+  lib,
+  ...
+}:
+# tmux for the primary user, configured through home-manager.
+# Settings without a home-manager option are read from ./extra.conf.
+let
+  cfg = config.modules.shell.tmux;
+  user = config.user.name;
+in
+{
+  options.modules.shell.tmux.enable = lib.mkEnableOption "tmux, configured via home-manager";
+
+  config = lib.mkIf cfg.enable {
+    registry.shell-shortcut.${config.networking.hostName}.tmx = "tmux new-session -A -s";
+
+    home-manager.users.${user}.programs.tmux = {
+      enable = true;
+
+      prefix = "C-Space";
+      keyMode = "vi";
+      mouse = true;
+      baseIndex = 1;
+      clock24 = true;
+      escapeTime = 10; # short Esc delay so exiting insert mode in nvim isn't laggy.
+      historyLimit = 10000;
+      terminal = "tmux-256color";
+
+      extraConfig = builtins.readFile ./extra.conf;
+    };
+  };
+}