Add setting to hide built-in git status line

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using a custom statusLine in settings.json, the built-in git status display (e.g., "3 files +18 -18") still appears alongside it. This creates redundant information when the custom statusline already shows git diff stats.

Current behavior:

my-project  on  main  ∙  +100 -232       ← custom statusline
████████░░ 82% left                      ← custom statusline  
3 files +18 -18                          ← built-in (can't hide)

Proposed Solution

Add a setting to hide the built-in git status when using a custom statusline:

{
  "statusLine": {
    "type": "command",
    "command": "~/.claude/statusline.sh"
  },
  "hideBuiltInStatus": true
}

Or more granular control:

{
  "builtInStatus": {
    "gitChanges": false,
    "fileCount": false
  }
}

Alternative Solutions

• Could also be a flag within the statusLine config itself:

{
  "statusLine": {
    "type": "command",
    "command": "~/.claude/statusline.sh",
    "replaceBuiltIn": true
  }
}

Priority

Low - Minor UX improvement

Feature Category

CLI settings and configuration

Use Case Example

Users with custom statuslines that already display git information want full control over what's displayed, without duplication from the built-in status.

Additional Context

Related to #8832 (Full Customizable Status Bar) but specifically about hiding existing elements rather than adding new ones.