copilot --resume creates an extra empty session-state directory with a new session ID

### Describe the bug

When resuming a session with:

copilot --resume=

the original session is resumed correctly, but Copilot also creates a new session-state directory under ~/.copilot/session-state/ with a different session ID.

Example

- resumed/original session: f3db8a64-0d62-47a6-9253-dd005738ba5c
- extra new directory created: f35d8fc1-f713-4ecd-bad6-ca09a07cb1f6

Actual behavior

- the original session continues correctly
- agent/tool activity is still associated with the original session
- but a second session-state directory is created for the new ID
- important files inside that new directory are empty

Expected behavior

1. resume should reuse only the original session-state directory, with no extra new directory created

Why this seems wrong

I only noticed this because I built a macOS app to inspect Copilot sessions. From that view, it looks like resume creates a duplicate/empty session artifact even though the real resumed work belongs to the original session.

Repro steps

1. Start a Copilot CLI session and note its session ID
2. Exit the session
3. Run:
copilot --resume=
4. Observe: - original session is resumed
- a second new directory appears under ~/.copilot/session-state/
- files in the new directory are empty

Environment

- Copilot CLI version: 1.0.64
- OS: macOS

### Affected version

GitHub Copilot CLI 1.0.64

### Steps to reproduce the behavior

1. Start a Copilot CLI session and note its session ID
2. Exit the session
3. Run:
copilot --resume=
4. Observe: - original session is resumed
- a second new directory appears under ~/.copilot/session-state/
- files in the new directory are empty

### Expected behavior

Expected behavior

1. resume should reuse only the original session-state directory, with no extra new directory created

### Additional context

Environment

- Copilot CLI version: 1.0.64
- OS: macOS

Tái hiện hành vi bằng copilot --resume= trên macOS và kiểm tra ~/.copilot/session-state/ trước và sau khi tiếp tục. Theo dõi cách ID phiên ban đầu và ID phiên mới được chọn và lưu bền vững; hoàn tất khi resume chỉ sử dụng lại thư mục ban đầu và không tạo thư mục thứ hai rỗng.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.