claude-teams: pane splits ignore layout, steal focus, and ignore resize

[lawrencecchen]

cmux claude-teams teammate panes don't respect the pane they were launched from.

Claude's agent teams protocol sends split-window -h for each teammate, then select-layout main-vertical to rearrange into a left-main + right-column pattern. Three things are broken:

Layout: select-layout main-vertical is silently ignored (no-op at line 9772 of cmux.swift). Repeated horizontal splits of the leader pane create side-by-side panes instead of stacking vertically in a right-side column. Expected: leader on left, teammates stacked vertically on the right.

Focus: surface.split calls newSplit(focus: true) by default, so internal bonsplit focus moves to each new teammate pane. Focus should stay on the leader pane.

Resize: resize-pane -x 30% without directional flags (-L/-R/-U/-D) is treated as a no-op, so the leader pane never gets resized to the intended width.

Repro: have a top + bottom pane, run cmux claude-teams in the top pane, let Claude spawn 2+ teammates. Observe panes spread horizontally instead of forming a vertical column to the right of the leader.