home: wm: i3: launch tmux when starting terminal

Similarly to the `zsh` snippet, launch tmux unless I explicitly do not
want it.

The reason I also add it in `i3` is to make the launch happen sooner,
and to avoid doing double work on some shell startup shenanigans if I
can.

home/wm/i3/default.nix

@@ -157,7 +157,9 @@ in
         keybindings = lib.my.recursiveMerge [
           {
             # The basics
-            "${modifier}+Return" = "exec ${terminal}";
+            "${modifier}+Return" = "exec ${terminal} ${
+              lib.optionalString config.my.home.tmux.enable "-e tmux new-session"
+            }";
             "${modifier}+Shift+Return" = "exec env TMUX=nil ${terminal}";
             "${modifier}+Shift+q" = "kill";
             "${modifier}+f" = "fullscreen toggle";

home/zsh/default.nix

@@ -46,6 +46,16 @@ in
     # Modal editing is life, but CLI benefits from emacs gymnastics
     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
+      ''
+    ;
+
     initExtra = lib.concatMapStrings builtins.readFile [
       ./completion-styles.zsh
       ./extra-mappings.zsh