[FEATURE] Agent Teams tmux mode: make the leader pane width configurable and keep user resizes

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In Agent Teams with teammateMode: "tmux", the leader pane is reduced to 30% of the window width as soon as the first teammate spawns, and it is reset to 30% every time another teammate spawns.

The leader pane is the one I type into and read results from, so it is the pane I use most. On a ~200-column terminal, 30% leaves about 60 columns, which is too narrow to read code, diffs, or long tool output comfortably.

Resizing the leader pane by hand does not last: the next teammate spawn sets it back to 30%. Setting tmux's main-pane-width has no effect either, because the width is applied with an explicit resize-pane.

From the bundled code in 2.1.285 (TmuxBackend), the behavior comes from:

• first teammate: split-window -d -t <leader> -h -l 70% (the new pane takes 70%, the leader keeps 30%)
• every later teammate (rebalancePanesWithLeader): select-layout main-vertical, then resize-pane -t <first pane> -x 30%

Proposed Solution

Either of these would solve it; both together would be best:

1. A setting for the leader pane width, for example "teammateLeaderPaneWidth": "50%" in settings.json (default 30% to keep the current behavior).
2. Keep the leader's current width when rebalancing: apply the 30% default only when the first teammate is spawned, and let later rebalances resize only the teammate panes, so a manual resize survives new spawns.

Alternative Solutions

• teammateMode: "in-process": avoids the problem but loses the per-teammate panes.
• Starting the leader outside tmux, so teammates go to a separate tiled window: works, but the leader is then not inside tmux.
• Resizing the leader by hand after all teammates have spawned: works until the next spawn resets it.

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

Replaying the tmux commands above in a 200x50 window:

| Step | Leader pane width |
|---|---|
| leader alone | 200 |
| after teammate 1 | 59 |
| after teammate 2 | 60 |
| user runs resize-pane -x 120 on the leader | 120 |
| after teammate 3 | 60 (manual resize lost) |

With a width setting of 50%, or with the manual width preserved, I could keep reading the leader's output while teammates run.

Additional Context

• Claude Code 2.1.285, tmux 3.7c, macOS
• Related: #23615 (spawn teammates in a new window), #25396 (teammates as separate tmux windows), #71799 (teammate grid layout), #23950 (split direction). None of these covers the leader pane width.