[FEATURE] Option to disable built-in git diff summary in status line

Problem

When using a custom statusLine command (like cc-quadstat), Claude Code still displays its built-in git diff summary below the custom output, causing duplicate information:

   Opus 4.5  v2.1.9  ⎇ r (+9930,-7)      <- custom statusLine (shows diff)
   🧠 72%  ⏱ 5h: 8% (2h44m) 
   📅 All: 18% (火19:59)  💬 Sonnet: 1% (水3:59) 
  21 files +9930 -7                       <- Claude Code built-in (duplicate)

Proposed Solution

Add a setting to disable the built-in git diff summary when a custom statusLine is configured:

{
  "statusLine": {
    "type": "command",
    "command": "my-custom-statusline",
    "showGitDiff": false
  }
}

Or a global setting:

{
  "showGitDiffInStatusLine": false
}

Use Case

Users who create custom status lines often include their own git information. The built-in display creates visual clutter and redundant information.

Environment

• Claude Code version: v2.1.9
• OS: macOS