Workflows marked removed/manual persist in UI after restart; no supported hard-delete or purge path Body

## Summary

Workflows that were intentionally disabled/removed still persist in the GitHub Copilot app UI after restart, and there does not appear to be a supported way to permanently delete them.

This seems like both:
1. a bug in workflow persistence / UI state, and
2. a product gap because there is no true delete/purge action.

Related issue: #1045

## Expected behavior

After workflows are disabled/removed, I would expect one of these to be true:

1. They disappear from the app UI, or
2. The app clearly indicates they are archived/disabled and provides a supported hard-delete/purge option.

## Actual behavior

The workflows remain visible in the app even after all of the following:

- setting every workflow to `manual`
- renaming them with a `[REMOVED ...]` prefix
- restarting the app
- resetting local app/WebView state by renaming:
- `Local Storage`
- `Session Storage`
- `Preferences`
- `Secure Preferences`

After reopening the app, the workflows still reappear.

## Lifecycle gap

The app/tooling supports workflow creation and modification, but there does not appear to be an equivalent supported remove/delete operation.

In practice, this means workflows can be:
- created easily
- edited easily
- disabled manually

But they cannot be fully removed from the UI/state in a supported way.

That makes the workflow lifecycle incomplete and leaves users with persistent records they can no longer clean up properly.

## Why this seems like a bug

This suggests the workflow list is being rehydrated from another source of truth, but the app does not expose that behavior clearly and does not provide a supported way to fully remove workflow records.

From the user perspective, the workflows appear "stuck":
- they are no longer intended to run
- they were explicitly set to manual
- local cache/state was reset
- but they still persist in the UI

## Impact

- cluttered workflow/automation UI
- confusion about whether workflows are actually gone
- concern that old automations may still affect app performance or state
- no practical cleanup path after creating many experimental workflows

## Requested fix

Please add one or both of these:

1. A true workflow delete/purge action
2. Clearer UX that distinguishes:
- active workflows
- manual/disabled workflows
- archived workflows
- deleted workflows

If hard delete is intentionally unsupported, the app should say so explicitly and explain what "removed" actually means.

## Environment

- OS: Windows
- App: GitHub Copilot app
- Context: large number of workflows created for experimentation/testing

## Additional notes

I was able to neutralize the workflows operationally by setting them all to `manual`, but I was not able to make them disappear from the UI.

This makes it hard to tell whether they are truly inactive versus just visually lingering.

Không có tệp, bài kiểm thử hoặc điểm vào triển khai nào được nêu. Tái hiện hành vi trong Windows GitHub Copilot app bằng cách đặt workflows thành manual, khởi động lại và đặt lại trạng thái cục bộ được liệt kê; được coi là hoàn tất khi các workflows đã xóa không còn xuất hiện hoặc UI xác định rõ trạng thái của chúng và cung cấp hành vi purge được hỗ trợ.

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