[FEATURE] Configurable mode cycling via modeCycle setting

Problem

Shift+Tab cycles through all four modes: plan → default → acceptEdits → bypassPermissions. For users whose workflow only uses a subset (e.g., plan and bypassPermissions), cycling through unused modes adds unnecessary friction.

This was previously requested in #8292 (closed as stale), but the demand is still there.

Proposed Solution

Add a modeCycle setting in settings.json to restrict which modes Shift+Tab cycles through:

{
  "modeCycle": ["plan", "bypassPermissions"]
}

• Default behavior unchanged: omitting the setting cycles through all modes as today.
• Validation: only accept valid mode names; ignore or warn on invalid entries.
• Minimal scope: this only affects the Shift+Tab keyboard shortcut — all modes remain accessible via / commands.

Use Case

My workflow is almost exclusively plan → bypassPermissions. I never use acceptEdits or default, so cycling through them is pure friction. A configurable cycle list would be a small, low-risk change that respects existing defaults while letting power users streamline their flow.