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.

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