Keybindings: direct model-select and effort-adjust actions (not just open-picker)

Currently keybindings.json only exposes chat:modelPicker (opens the picker) and modelPicker:increaseEffort/decreaseEffort (scoped to when the picker is already open). There's no way to bind a single key that:

1. Switches directly to a named model (e.g. Sonnet, Opus) without opening/navigating the picker, or
2. Adjusts effort level from the chat input without the picker open.

Requested: standalone actions like chat:setModel:<name> (or a cycle action) and chat:increaseEffort/chat:decreaseEffort usable directly from the Chat context, for users who want one-keystroke model/effort switching.