Keyboard focus for the diff/review panel (no way to "enter" it)

**Problem:** When a diff/review ("proposed changes") panel is showing, keystrokes always go to the prompt input. There's no way to move keyboard focus into the diff panel, so its navigation keys (`j`/`k`, arrows, `space`/`b`, `g`) never apply unless the panel was opened modally via `/diff`. If the diff is just displayed alongside the prompt, it can't be driven from the keyboard at all — you have to use the mouse.

**Request:** A focus-toggle / cycle-pane keybinding (e.g. `Tab` or `Ctrl+O`) that moves focus between the prompt and the diff panel, exposed as bindable commands in `~/.claude/keybindings.json` (e.g. `app:focusDiffPanel` / `app:focusChat`, or a `app:cyclePane`). Today `keybindings.json` has `DiffDialog` / `DiffPanel` contexts but no focus/cycle command, so it can't be worked around by rebinding.

**Environment:** Claude Code CLI (terminal).

Start with ~/.claude/keybindings.json and the DiffDialog/DiffPanel contexts, then trace how /diff opens the panel and how the prompt currently receives keystrokes. Add a bindable focus or cycle command so focus can move between the prompt and displayed diff panel, and verify that the diff navigation keys work in both panes without modal opening.

Scritto dal modello di indicizzazione a partire dal testo della issue.