When sending messages to Claude sessions via tmux, the Enter key send
could fail silently. This caused polecats to receive their initial
prompt but never submit it - the message appeared in Claude's input
area but Enter was never pressed.
Add retry logic (up to 3 attempts with 200ms delays) for the Enter
send step in both NudgeSession() and NudgePane(). This ensures message
submission is more reliable even if tmux has transient issues.
Fixes#41