[Feature Request] Custom modes in Shift+Tab cycle with config-based behavioral instructions

Question

**Bug Description**
Feature request: Custom modes in the Shift+Tab cycle

It would be useful to define custom named modes (e.g. a "mentor" mode, a "review-only" mode) that slot into the Shift+Tab mode switcher
  alongside the built-in ones. Each custom mode would be defined in a config or markdown file and could specify behavioral instructions and/or
  permission rules. The closest workaround today is a custom slash command, but integrating into the mode cycle would make it more ergonomic for