`edit` tool adds UTF-8 BOM (`EF BB BF`) to files on Windows

### Describe the bug

On Windows, Copilot CLI's `edit` tool prepends a UTF-8 Byte Order Mark (`\xef\xbb\xbf`)
to the output file after a modification, even when the original file had no BOM.

### Affected version

v1.0.49

### Steps to reproduce the behavior

**Steps to reproduce**

1. Create a UTF-8 file without BOM (e.g., via PowerShell with explicit UTF-8 no-BOM write)
2. Use the `edit` tool to replace any `old_str` with a `new_str`
3. Read the output file's first 3 bytes:
```powershell
$b = [IO.File]::ReadAllBytes("test.md")
"{0:X2} {1:X2} {2:X2}" -f $b[0], $b[1], $b[2]
# Expected: first 3 bytes of actual content (e.g., "23 20 74" for "# t")
# Actual: EF BB BF ← UTF-8 BOM
```

### Expected behavior

no edit failure

### Additional context

**Evidence (v1.0.49, Windows 11 / PowerShell 7, 2026-05-19)**

Byte-level measurement before and after `edit` tool call:

```
Before edit: CRLF=3 LF-only=0 bytes=35 BOM=false
After edit: CRLF=3 LF-only=0 bytes=42 BOM=true
Hex header: EF BB BF 6C 69 6E 65 20 6F 6E 65 0D 0A ...
```

The `edit` tool correctly normalizes line endings for `old_str` matching (LF `old_str`
matched CRLF file content successfully), but the **output file gains a BOM** and retains
CRLF — so the normalization only applies to matching, not to the written output.

**Impact**

- Python `open(encoding='utf-8')` reads the BOM as the `\ufeff` character prepended to
the file content, breaking JSON parsers, YAML parsers, and string-prefix matching.
- `git diff` displays BOM bytes as binary noise, cluttering diffs.
- `.gitattributes eol=lf` does **not** strip BOM — git treats it as opaque file content.
Without a dedicated pre-commit hook, BOM is committed to the repository permanently.

**Requested change**

Use `new UTF8Encoding(encoderShouldEmitUTF8Identifier: false)` (or equivalent) when
writing files on Windows, so that the output is BOM-free UTF-8 — the format expected by
virtually all modern tooling and consistent with behavior on macOS/Linux.

**Related**

- #1148 — CRLF injection (same root cause area: Windows file API defaults)

**Workaround (current)**

After using the `edit` tool, re-write the file with Python to strip BOM:
```python
with open(path, encoding='utf-8-sig') as f:
content = f.read() # strips BOM
with open(path, 'w', encoding='utf-8', newline='\n') as f:
f.write(content)
```

首先，在 Windows 上使用不含 BOM 的 UTF-8 檔案重現編輯操作，然後檢查編輯工具在 Windows 上的檔案寫入路徑。當修改後的檔案仍不含 BOM，同時保留其內容和現有的 CRLF 換行字元時，修改即完成；使用 issue 中的 PowerShell 檢查驗證前三個位元組。

由索引模型根據 Issue 內容生成。