Updates fields on a task.

## Usage

```json
{"id": "at-123.1", "status": "in_progress"}
{"id": "at-123.1", "status": "completed", "summary": "Fixed the bug"}
{"id": "at-123.1", "status": "completed", "summary": "Found 5 issues", "facts": ["All use deprecated API"], "next_steps": ["Migrate to new API"]}
```
