Add setting to disable/hide companion buddy

Summary

There should be a setting to disable or hide the companion buddy (the small creature next to the input box). Not everyone wants it in their workflow.

Suggested implementation

A simple toggle in ~/.claude/settings.json:

{
  "companionEnabled": false
}

Or alternatively via a CLI flag / /config option.

Context

• Several issues have been opened today requesting customization (#41942, #41990, #41844, #42100, etc.), but none specifically for a disable toggle.
• This is a basic UX preference — power users who want a minimal interface should be able to opt out.