fix(tmux): use tmux-256color (not tmux-direct); add NO_TMUX hatch

tmux-direct as default-terminal desyncs zsh's line redraw on some
terminals (iTerm2: duplicated characters on Tab, stray newlines). Switch
to the standard tmux-256color and advertise truecolor per outer terminal
via terminal-features (add xterm-256color:RGB alongside the foot ones).

Also add a NO_TMUX escape hatch to the auto-start guard, so
`NO_TMUX=1 <terminal>` opens a bare shell.

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

lyrathorpe/home/shell.nix

@@ -66,9 +66,11 @@
       #   not SSH          -> don't force inbound SSH logins into a server tmux
       #   not VS Code      -> its integrated terminal manages itself
       #   tmux on PATH     -> a failed exec would otherwise kill the login shell
+      #   $NO_TMUX unset   -> escape hatch: `NO_TMUX=1 <term>` opens a bare shell
       (lib.mkOrder 200 ''
         if [[ $- == *i* ]] \
           && [[ -z "$TMUX" ]] \
+          && [[ -z "$NO_TMUX" ]] \
           && [[ -z "$SSH_CONNECTION" && -z "$SSH_TTY" ]] \
           && [[ "$TERM_PROGRAM" != "vscode" ]] \
           && command -v tmux >/dev/null 2>&1; then
@@ -139,7 +141,11 @@
   programs.tmux = {
     enable = true;
     reverseSplit = true;
-    terminal = "tmux-direct";
+    # tmux-256color (not tmux-direct): the standard inside-tmux terminfo.
+    # tmux-direct's capabilities desync zsh's line redraw on some terminals
+    # (e.g. iTerm2 -> duplicated chars on Tab, stray newlines). Truecolor is
+    # advertised per outer terminal via the RGB terminal-features below.
+    terminal = "tmux-256color";
     newSession = true;
     keyMode = "vi";
     historyLimit = 500000;
@@ -187,6 +193,10 @@
       bind -n M-Up select-pane -U
       bind -n M-Down select-pane -D
 
+      # Truecolor for the outer terminals (foot reports xterm-ish too; iTerm2 is
+      # xterm-256color). Without this, with tmux-256color as default-terminal,
+      # 24-bit colour would be quantised to 256.
+      set -as terminal-features ",xterm-256color:RGB"
       # Tell tmux which capabilities the foot terminal supports, so truecolor,
       # synchronised output, the system clipboard (OSC 52), window titles and
       # cursor styling all pass through.