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"

Start by reproducing the issue with CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=true, teammateMode: "tmux", and pane-border-status off in the tmux/psmux configuration. Trace the teammate-spawn path and verify that a pane title can still be shown while pane-border-status remains off, including after multiple teammate spawns.

索引モデルが issue の本文から書いたものです。