`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)
```

Commencez par reproduire l’opération de modification sous Windows avec un fichier UTF-8 sans BOM, puis examinez le chemin d’écriture des fichiers de l’outil de modification sous Windows. La modification est terminée lorsque le fichier modifié reste dépourvu de BOM tout en préservant son contenu et ses fins de ligne CRLF existantes ; vérifiez les trois premiers octets avec la vérification PowerShell de l’issue.

Rédigé par le modèle d'indexation à partir du texte de l'issue.