feature: tmux-agents v1.4.0 visible splits
I shipped v1.4.0 of tmux-agents so spawned agents can open in a visible split with tmux-spawn --split <pane> [--right | --below] [--size N%]. It came from the layout I wanted for my agent chain: main on the left half, secondary top right and worker bottom right, all in one window, using --split main --right for secondary and --for secondary --split secondary --below for the worker. Split agents keep the agent list, status chip, messaging, closing and reopening, though reopened agents return in hidden windows. The decisions are recorded, and tests/spawn-split.sh covers the feature with 43 checks.



