PRD 0021 follow-up. The new-agent flow was calling a dedicated
`_tmux_split_pane_tail` that ALWAYS created a new pane —
so every `n` start spawned a fresh right pane next to any
existing one, accumulating panes instead of reusing them.
Replace with a generic `_ensure_right_pane(tmux_state, argv)`
that respawns the dashboard's tracked right pane if one is
alive, splits a new one only when none is tracked or the
tracked pane was closed. Both the new-agent tail-during-
bringup path AND the existing claude-attach path now route
through this helper.
Net effect: starting a second agent reuses the same right
pane — bringup tail replaces the prior claude session,
then claude (for the new agent) replaces the tail. Closing
the right pane manually via `C-b x` still triggers a fresh
split on the next attach.
didericis
0936c40428