Reorder right-panel tabs by dragging

[andybergon]

Problem or use case

Right-panel surfaces (Files, Diff, Browser, Terminal, Agents, Pull request) render in the
order they happened to be opened, and there is no way to change that order afterwards.

The order is not cosmetic. The tab context menu already offers Close others,
Close to the right, and Close all, so Close to the right acts on a sequence the
user cannot control. Reopening a surface to move it also destroys its state: a browser
tab loses its page, a terminal loses its session.

Proposed solution

Allow dragging a right-panel tab to a new position in the tab strip, with the order
persisted per thread alongside the rest of the panel state.

Why this matters

Diff and Files are the two surfaces I keep coming back to, but they end up wherever they
were opened, often behind three browser tabs. Every tabbed interface I use, including this
app's own model picker after #6743, lets me put the important thing first.

Smallest useful scope

Drag to reorder within the existing tab strip, persisted per thread. No cross-panel drag,
no detaching a tab into its own window, no pinning.

Alternatives considered

• Close and reopen to change position: loses browser page and terminal session state.
• Per-project default tab order ([Feature]: Add per-project default right panel tabs for new threads #6829): sets the starting layout, but does not help once
  a thread is in progress.

Risks or tradeoffs

A persisted order has to survive a surface disappearing, for example a closed browser tab
or a workspace that is no longer available, without resorting the tabs that remain.