chat:cycleMode — allow configuring which modes are included in the Tab cycle

Currently shift+tab cycles through all 5 modes in fixed order: bypass, auto, nothing, accept edits, plan. There's no way to limit the cycle to a subset via keybindings.json or settings.

Request: Allow users to configure which modes are included in the cycle — e.g., to cycle only between plan and bypass, skipping the other three.

Use case: some workflows only ever use 2 modes and cycling through all 5 adds unnecessary friction.