[FEATURE] Add a bindable keybinding action to toggle focus mode (`/focus`)

What I'm trying to do

Toggle focus mode (the /focus view) with a keyboard shortcut, without having to type the /focus slash command each time.

The gap

The keybindings reference exposes bindable toggle actions for several view states in the Global/Chat contexts, e.g.:

• app:toggleTranscript → Ctrl+O (toggle verbose transcript)
• app:toggleTodos → Ctrl+T (toggle task list)
• chat:fastMode → Meta+O (toggle fast mode)
• chat:thinkingToggle → Meta+T (toggle extended thinking)

But there is no action to toggle focus mode. Focus mode can only be entered/exited via the /focus slash command, so it can't be bound to a key in ~/.claude/keybindings.json.

Request

Add a bindable action — e.g. app:toggleFocus (or chat:toggleFocus) — that toggles focus mode the same way app:toggleTranscript toggles the transcript. Leaving it unbound by default is fine; the point is to make it bindable via the keybindings config.

Why it's useful

Focus mode is something you flip in and out of frequently during a session. A togglable shortcut (consistent with the existing Ctrl+O / Ctrl+T toggles) is much faster than typing /focus repeatedly, and lets users pick a binding that fits their terminal/muxer setup.