feat(shell): panes start fish via default-command

tmux.conf

@@ -96,6 +96,11 @@ bind -N "Previous session" -n M-Up switch-client -p
 bind -N "Next session" -n M-Down switch-client -n
 
 # General
+# Panes run fish, the primary interactive shell, while the login shell (and so
+# tmux's default-shell, which runs this and every run-shell/#() job) stays
+# bash. Falls back to the login shell where fish is missing (kitty's ssh kitten
+# copies this config to remote hosts). The line parses in sh and fish alike.
+set -g default-command 'command -v fish >/dev/null 2>&1 && exec fish -l || exec "$SHELL" -l'
 set -g default-terminal "tmux-256color"
 set -ag terminal-overrides ",*:RGB"
 set -g mouse on