home: zsh: disable tmux auto-launch by default

I'm about to flip the default to always disabled, so enable it
explicitly.

home/zsh/default.nix

@@ -3,8 +3,10 @@ let
   cfg = config.my.home.zsh;
 in
 {
-  options.my.home.zsh = with lib.my; {
-    enable = mkDisableOption "zsh configuration";
+  options.my.home.zsh = with lib; {
+    enable = my.mkDisableOption "zsh configuration";
+
+    launchTmux = mkEnableOption "auto launch tmux at shell start";
   };
 
   config = lib.mkIf cfg.enable {
@@ -44,14 +46,16 @@ in
       defaultKeymap = "emacs";
 
       # Make those happen early to avoid doing double the work
-      initExtraFirst =
-        lib.optionalString config.my.home.tmux.enable ''
-          # Launch tmux unless already inside one
-          if [ -z "$TMUX" ]; then
-            exec tmux new-session
-          fi
-        ''
-      ;
+      initExtraFirst = ''
+        ${
+          lib.optionalString cfg.launchTmux ''
+            # Launch tmux unless already inside one
+            if [ -z "$TMUX" ]; then
+              exec tmux new-session
+            fi
+          ''
+        }
+      '';
 
       initExtra = ''
         source ${./completion-styles.zsh}

machines/porthos/default.nix

@@ -5,6 +5,7 @@
   imports = [
     ./boot.nix
     ./hardware.nix
+    ./home.nix
     ./networking.nix
     ./services.nix
     ./users.nix

machines/porthos/home.nix

@@ -0,0 +1,7 @@
+{ ... }:
+{
+  my.home = {
+    # Always start a tmux session when opening a shell session
+    zsh.launchTmux = true;
+  };
+}