[TMUX] Pass Shift+Enter through as CSI-u

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tmux/.tmux.conf

@@ -26,6 +26,8 @@ bind -T off F12 \
 
 setw -g mode-keys vi
 set-option -g status-keys vi
+set -g extended-keys on
+set -g extended-keys-format csi-u
 
 # Splits keep the source pane's cwd AND its active Python venv. #{@venv} is set
 # per-pane by the shell (see .virtual_env_config.zsh); we go through run-shell