Allow suppressing or customizing the PR number in the built-in mode indicator

Summary

The built-in mode indicator line (e.g. ⏵⏵ accept edits on (shift+tab to cycle) · PR #1234) always appends the current branch's associated PR number when one exists. There is no way to suppress just the PR portion.

Use case

When using a custom statusLine command (via settings.json) that already surfaces PR context — for example, showing session-active PRs across multiple repos — the PR #NNNN in the built-in indicator is redundant and adds noise.

Requested change

Add a settings.json option to control whether the PR number is shown in the built-in mode indicator. Something like:

{
  "modeIndicator": {
    "showPR": false
  }
}

Or alternatively, allow the entire mode indicator line to be replaced by a custom command (similar to how statusLine works), so users have full control over both lines.

Current workaround

None — the PR number cannot be suppressed today without removing PR context from the custom statusline entirely, which defeats the purpose of having both lines.