Auto-request reviewers via GitHub API

## Feature Request

Add the ability to automatically request reviewers on the PR, not just label and comment.

### Proposed behavior

- New input `auto-request: true/false` (default `false`)
- New input `reviewer-pool` — list of GitHub usernames to pick from
- Automatically request the recommended number of reviewers from the pool
- Skip the PR author from the pool
- Respect existing review requests (don't double-request)

### Example usage

```yml
- uses: anistark/pr-reviewer-labeler@v2
with:
auto-request: true
reviewer-pool: |
- alice
- bob
- carol
- dave
```

### Why

Labeling and commenting are informational — someone still has to manually assign reviewers. Auto-requesting closes the loop and makes the action fully actionable.