rightPanel.nextTab and rightPanel.previousTab, so Ctrl+Tab can cycle right-panel tabs

[andybergon]

Problem or use case

Ctrl+Tab does nothing in the right panel, and no other key switches right-panel tabs
either. The mouse is the only way to move between Diff, Files, a browser tab, and a
terminal.

This is not the chord-capture problem from #6966. The keybindings list has no next-tab or
previous-tab command for the right panel, so there is nothing to bind in the first place.
#6817 is the umbrella request for this class of missing command.

Proposed solution

Bindable "next right-panel tab" and "previous right-panel tab" commands, cycling the
current thread's tabs and wrapping at each end.

They would not need a default binding, the same as rightPanel.toggleMaximized, so anyone
who wants ctrl+tab and ctrl+shift+tab can set them in Settings > Keybindings. A default
would need a right-panel focus condition, and the when conditions available today are
terminalFocus, terminalOpen, previewFocus, previewOpen, and modelPickerOpen.

Why this matters

The right panel is where the work is actually read. #6791 asked for Cmd+W to close the
active surface and #6824 asked for a command to open a terminal there, so tab navigation is
the missing piece of the same set.

Smallest useful scope

Next and previous only, unbound by default. Numbered jumps can follow if there is demand.

Alternatives considered

• diff.toggle and preview.toggle already reach two right-panel surfaces by key, but
  they toggle one specific surface rather than cycling, and there is no equivalent for
  Files, Agents, Pull request, or a right-panel terminal. One command per surface kind
  scales worse than two cycle commands.
• Reordering tabs (Reorder right-panel tabs by dragging #7465) changes the order, not the navigation.
• The command palette searches threads, projects, branches, and messages. It does not
  switch right-panel surfaces.

Risks or tradeoffs

A focused browser or terminal receives keys first, so a ctrl+tab binding may need a
!terminalFocus guard.