feat: option to pin chat input at top of screen

Question

## Feature Request: Chat input position option (top vs bottom)

Currently the chat input is fixed at the bottom of the terminal. No configuration option exists to move it.

**Problem:** When output is long, input disappears off-screen. Users who prefer input-at-top (common in many chat UIs and editors) have no option. Heavy keyboard users lose context of what they typed vs what's being streamed.

**Request:** Add a config option:

```json
{ "inputPosition": "top" | "bottom" }
```

Output would s