rfc: force show/hide cursor option

## Context

Cursor rendering now respects terminal state automatically via `terminal.modes.showCursor`.

## Potential Enhancement

If users need explicit control to force cursor on/off regardless of terminal state, we could add a parameter:

```typescript
shell_screenshot({ session_id, showCursor?: 'auto' | 'show' | 'hide' })
```

- `auto` (default): respect terminal state
- `show`: always render cursor
- `hide`: never render cursor

## Use Cases

- Force show cursor in TUI apps that hide it
- Force hide cursor in shell sessions for cleaner screenshots

Please comment if you have a use case for this.