Tmux Renaming fails if pane idx isn't 0

Ran into an issue where uzi failed to spin up an agent via tmux session and died when it tried to rename the session

Root issue was that i have my default pane index starting at 1 instead of zero

The session renaming could be more robust (eg just rename the minimum pane idx) and/or have more robust error handling.

Also might be worth considering how uzi inherits the user tmux config and if it would make sense to have its own configuration to keep things clean.