Add a keyboard shortcut to expand a pane to full width

First off, the pane shortcuts are great and I use them constantly — ⌘⇧B for
the browser, ⌘J for the terminal, ⌘⇧D for the diff.

The gap is that they only open and close a pane. Once it's open there's no
way to make it bigger without grabbing the mouse and dragging the divider.

That's the part that breaks for me. I hit the shortcut to bring up the
browser, and then immediately have to reach for the mouse to actually see
anything in it. Every time.

What I'd love is a second keypress. One shortcut opens the pane, another
expands it to the full width of the window. Hit it again and it goes back.
That's all.

I went looking at what the View menu can actually send, and the pane
commands are only toggleSessionPane, closeSessionPane and splitViewCommand
— there's nothing for sizing. So I think this needs a new action, not just
a new binding.

The closest I can get today is hiding the sidebar (⌘B) and the side chat
(⌘;) to free up space. It helps, but that's three shortcuts and still not
full width.

Would really love to see this one land. macOS, Code tab.

Start by tracing the View-menu actions toggleSessionPane, closeSessionPane, and splitViewCommand in the macOS Code tab, then locate the pane sizing and shortcut entry points. Done means a new action can expand the pane to full window width and toggle it back without dragging, with the existing pane shortcuts still working.

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