Agent Teams (tmux mode) forces pane-border-status on, overriding user config

Problem

When Claude Code spawns a teammate in tmux/psmux split-pane mode, it sets the pane title
(as covered by #21677/#31107) - that part is fine and wanted. The problem is a side effect:
pane-border-status gets forced to "top" globally, even when the user has explicitly set it
to "off" (both live via set-option -g pane-border-status off and in .tmux.conf/.psmux.conf).

Confirmed this is NOT a tmux/psmux-side behavior: manually setting a pane title via
select-pane -T "test" on a non-teammate pane does not touch pane-border-status at all -
it stays off. Only teammate-spawned panes flip it on, and re-flip it on every new teammate
spawn even after being manually turned back off mid-session (no server restart involved).

Requested fix

Set the pane title without touching pane-border-status, or gate that call behind a
settings.json flag independent of the title-setting behavior in #21677/#31107 - some users
want the title shown without a border/header line.

Environment

• Windows 11, psmux (tmux-compatible multiplexer) as teammateMode: "tmux" backend
• Requires CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=true (Agent Teams / tmux mode is gated

behind this experimental flag)

• Reproducible: set pane-border-status off, spawn a teammate, observe it reset to "top"