feat(tmux): auto-start in graphical terminals

Opening a terminal (foot) execs `tmux new-session -A -s main`, so every new
terminal lands in the multiplexer; panes run a plain non-login zsh. Guarded
to interactive, not-already-in-tmux, not-SSH, not-VS-Code, tmux-present --
preventing re-exec loops, hijacked scp/ssh shells, and lockout. Lives in the
graphical desktop layer, so the WSL work box keeps a plain shell.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyrathorpe/home/desktop.nix

@@ -6,6 +6,7 @@
   pkgs,
   config,
   inputs,
+  lib,
   username,
   ...
 }:
@@ -95,4 +96,25 @@
       };
     };
   };
+
+  # Auto-start tmux in graphical terminals (foot): opening a terminal drops
+  # straight into a session named "main" (attach if it exists, else create).
+  # Panes inside run a plain non-login zsh (tmux's default-command "${SHELL}").
+  # Order 200 runs this before oh-my-zsh/compinit so the exec replaces the shell
+  # before that setup is wasted. Guards (each prevents a real breakage):
+  #   interactive only   -> don't hijack scp/ssh-command/non-interactive shells
+  #   $TMUX empty        -> a pane's zsh won't re-exec tmux (infinite loop)
+  #   not SSH            -> don't force inbound SSH logins into a server tmux
+  #   not VS Code        -> its integrated terminal manages itself
+  #   tmux on PATH       -> exec failure would otherwise kill the login shell
+  # Lives here (graphical hosts only); the WSL work box never imports this.
+  programs.zsh.initContent = lib.mkOrder 200 ''
+    if [[ $- == *i* ]] \
+      && [[ -z "$TMUX" ]] \
+      && [[ -z "$SSH_CONNECTION" && -z "$SSH_TTY" ]] \
+      && [[ "$TERM_PROGRAM" != "vscode" ]] \
+      && command -v tmux >/dev/null 2>&1; then
+      exec tmux new-session -A -s main
+    fi
+  '';
 }