Add "Don't ask again" to delete session confirmation dialog

[Copilot]

Description

The confirmation dialog shown when deleting a session ("Are you sure you want to delete this chat session?") had no way to opt out of future prompts. This adds a "Do not ask me again" checkbox, reusing the same pattern already established by the archive-section actions.

• agentSessionsActions.ts (DeleteAgentSessionAction) — the dialog from the issue. Adds the checkbox; the choice is persisted via the new chat.sessions.confirmDelete profile storage key and skips the dialog on subsequent deletions.
• sessionsViewActions.ts (DeleteSessionAction) — the Agents window sessions view delete action. Same behavior, backed by the sessions.confirmDelete profile storage key.

Both mirror the existing ConfirmArchiveStorageKey flow: persist only when the user checks the box and confirms, at StorageScope.PROFILE / StorageTarget.USER, reusing the shared doNotAskAgain label.

const skipConfirmation = storageService.getBoolean(ConfirmDeleteStorageKey, StorageScope.PROFILE, false);
if (!skipConfirmation) {
    const confirmed = await dialogService.confirm({
        message: /* ... */,
        detail: localize('deleteSession.detail', "This action cannot be undone."),
        primaryButton: localize('deleteSession.delete', "Delete"),
        checkbox: { label: localize('doNotAskAgain', "Do not ask me again") }
    });
    if (!confirmed.confirmed) {
        return;
    }
    if (confirmed.checkboxChecked) {
        storageService.store(ConfirmDeleteStorageKey, true, StorageScope.PROFILE, StorageTarget.USER);
    }
}

Note: the "move to trash / restore" enhancement mentioned in the issue is out of scope here.