fix(tmux): stop clip.exe mangling non-ASCII in the WSL clipboard

tmux-yank autodetects WSL and pipes the selection to clip.exe, which
decodes its stdin as the console OEM codepage rather than UTF-8. Copying
an em dash out of a pane put "ΓÇö" on the Windows clipboard; the same
applies to every non-ASCII character.

Override the copy command to route through tmux's own buffer. With
set-clipboard on, that emits OSC 52 and the terminal receives UTF-8
directly, with no Windows-side helper in the path. Windows Terminal
honours OSC 52; verified against the running client.

Guarded on /proc/version so only WSL is affected. iTerm2 does not accept
OSC 52 by default, so the Darwin hosts keep pbcopy.

Set in the plugin's extraConfig rather than the shared block because
yank.tmux bakes the copy command into its key bindings at load time, and
home-manager emits plugin extraConfig before the run-shell.

home/shell.nix

@@ -287,7 +287,22 @@ in
     plugins = with pkgs.tmuxPlugins; [
       sensible
       vim-tmux-navigator # Ctrl-h/j/k/l across vim splits and tmux panes
-      yank
+      {
+        # On WSL, tmux-yank pipes the selection to clip.exe, which decodes its
+        # stdin as the OEM codepage instead of UTF-8 -- an em dash reaches the
+        # Windows clipboard as three characters. Route through tmux's own
+        # buffer instead: with set-clipboard on, tmux emits OSC 52 and the
+        # terminal takes the text as UTF-8. Windows Terminal honours OSC 52;
+        # iTerm2 does not by default, hence the runtime guard rather than
+        # overriding pbcopy/xsel on every host. yank.tmux bakes the command
+        # into its key bindings when it loads, so this must be set first, which
+        # is what plugin extraConfig gives us.
+        plugin = yank;
+        extraConfig = ''
+          if-shell 'grep -qi microsoft /proc/version 2>/dev/null' \
+            "set -g @override_copy_command 'tmux load-buffer -w -'"
+        '';
+      }
       extrakto # prefix+Tab: fzf-grab paths/URLs/text from the pane into the prompt
       {
         # Catppuccin Mocha statusline (v2 API: flavour + window options must be