Feature: stck sync --push to sync and push in one command

## Suggestion

The most common workflow is: fix something on an early branch, then sync the stack and push all branches. This always requires two commands:

```bash
stck sync
stck push
```

A `--push` flag on `stck sync` would streamline this:

```bash
stck sync --push
```

This combines the rebase cascade with the force push in a single step.