#20143 fix: restore pane ellipsis split actions

Summary

Restore Split Right and Split Down to the pane ellipsis menu. The agent composer header menu remains split-free; its split controls remain on the dedicated header actions, keyboard bindings, and drag targets.

Changelog

Fixed: restore pane ellipsis Split Right and Split Down actions.

Verification

• git diff --check
• Confirmed the composer ChatHeaderTools menu rows contain no split actions.
• No new unit test added per cmux-next policy; hosted Swift CI is the landing gate.

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Summary by cubic

Restores the Split Right and Split Down actions to the pane ellipsis menu.

• Removes the filter that excluded these actions for new tabs.
• The agent composer header menu is unchanged; its split controls remain on the dedicated header actions, keyboard bindings, and drag targets.