Add option to disable built-in PR badge in status bar

Summary

When using a custom statusLine command, the Claude Code UI still renders a built-in PR badge (e.g. #1627) next to the status line. There is currently no way to opt out of this.

Use case

Users who already display branch information in their custom status line script have no need for the PR badge and would like to hide it to reduce noise in the status bar.

Proposed solution

Add a settings option such as:

{
  "statusLine": {
    "showPRBadge": false
  }
}

or a top-level flag like:

{
  "showPRBadge": false
}

This would allow users to suppress the built-in PR indicator when it duplicates information they already show, or simply when they prefer a cleaner status bar.