Undocumented `logs/security/.security-key` file created in every working directory on startup

# Description

Copilot CLI creates a `logs/security/.security-key` file in the current working directory as soon as a session starts. This happens in every directory, including a brand-new empty one, so it appears to be unconditional CLI startup behavior rather than something tied to a specific repo or project config.

I can't find this documented anywhere - not in the CLI config-directory reference, the command reference, or recent changelogs (checked back through v1.0.76). Filing this to understand what the file is for and whether it's expected to live in the project directory rather than `~/.copilot/`.

# Steps to reproduce

```bash
mkdir /tmp/test-copilot && cd /tmp/test-copilot && copilot
# exit the session
ls -la logs/security/
```

# Observed

```bash
harshit@Harshits-MacBook-Pro /tmp/test-copilot » ls -la logs/security
total 8
drwxr-xr-x@ 3 harshit wheel 96 Jul 31 10:52 .
drwxr-xr-x@ 3 harshit wheel 96 Jul 31 10:52 ..
-rw-------@ 1 harshit wheel 32 Jul 31 10:52 .security-key
```

The file contains 32 bytes of what appears to be binary/random data (confirmed via xxd), not human-readable text.

# Questions

1. What is this file used for (e.g., local encryption of session state, sandbox integrity verification, telemetry)?
2. Is it intentional that it's written under `/logs/security/` rather than under `~/.copilot/`? This means every project directory a user runs copilot in accumulates this file, which could easily get committed to version control if not gitignored.
3. Should this path be added to a default `.gitignore` suggestion, or documented in the config-directory reference alongside the other `~/.copilot/` contents?

# Environment

- Copilot CLI version: GitHub Copilot CLI 1.0.77
- OS: macOS 26.4 (25E246)
- Install method: I don't remember, I have been using copilot for a while now. The only change that happened is I recently updated the version.

在空目錄中執行提供的 mkdir、cd、copilot 和 ls 重現步驟，然後追蹤建立 logs/security/.security-key 的 CLI 啟動路徑。閱讀報告中提到的 CLI 設定目錄參考、命令參考和 changelog。完成的標準是確立該檔案的用途和預期位置，並將相關行為修正或記錄，同時提供任何 gitignore 指引。

由索引模型根據 Issue 內容生成。