Keyboard-first checkpoint revert picker

[spaansba]

Problem

Reverting a thread checkpoint is difficult to discover and requires mouse interaction. Keyboard-heavy workflows need a fast way to inspect earlier checkpoints and choose a safe restore point.

Idea

Add a bindable checkpoint revert picker that:

• opens from the chat view with a keyboard shortcut
• previews the selected restore point in the transcript
• shows file diff statistics when available
• confirms with Enter and cancels with Escape
• optionally supports reverting directly to the latest checkpoint

Native Undo and Redo should keep working while focus is inside inputs, textareas, contenteditable elements, or other editing controls.

Stopped and older threads should use their persisted workspace and provider binding when possible. Missing Git checkpoints and providers without conversation rollback should show a clear unavailable state.

A generic undo-after-revert action may need separate provider support.

Prototype: #10573

[olafura]

You could easilly add this with #10708 where you would be able to customise exactly how all interactions would work including extending the currently pretty capable keyboard navigation