Allow disabling built-in PR number display on status line

Problem

When using a custom statusLine script in settings.json that includes PR information, Claude Code's built-in PR #xyz line still appears as a separate line below the custom status line output. This causes duplication and there's currently no way to suppress it.

Example output:

my-project | [Opus 4.6 (1M context)] | feature/update-auth
  S: 0 | U: 2 | A: 0 | PR: #123 | Ctx: 12%
  PR #123

The third line (PR #123) is rendered by Claude Code automatically and cannot be controlled or hidden via the custom statusLine script.

Proposal

Add a setting (e.g., "showPR": false or similar) to disable the built-in PR display, so users with custom status lines can fully control how PR info is presented.

This would allow merging the PR number into the custom status line layout without duplication.