Allow configuring which permission modes appear in Shift+Tab cycle

Problem

The Shift+Tab permission mode cycle always includes default, acceptEdits, and plan. There's no way to remove modes from the cycle — only auto and bypassPermissions can be disabled entirely.

For users who only want to toggle between plan and bypassPermissions (similar to how Codex toggles between suggest and full-auto), the extra modes add friction — you have to tap through modes you never use.

Proposed Solution

Add an allowedModes setting (or similar) to settings.json that controls which modes appear in the Shift+Tab cycle:

{
  "permissions": {
    "defaultMode": "plan",
    "allowedModes": ["plan", "bypassPermissions"]
  }
}

Only the listed modes would appear when cycling with Shift+Tab. Unlisted modes would still be accessible programmatically or via CLI flags, just not in the UI cycle.

Use Case

Power users who have a clear two-mode workflow (e.g., plan for review → bypass for execution) and don't need the intermediate modes cluttering the toggle.

Current Workaround

None — default and acceptEdits cannot be removed from the cycle today.