feat: parsec merge --batch for merging multiple PRs sequentially

## Summary

When merging multiple PRs, each must be done one-by-one. A batch mode would automate sequential merge.

## Proposed Behavior

```bash
parsec merge --batch 68 72 114 121
```

Each PR merged in order with auto-rebase between.

## Context

Parallel development with 4+ PRs requires sequential merging, currently manual.