feat(stack): add --skip-mr-creation flag to stack sync command

What does this MR do?

Adds a --skip-mr-creation flag to glab stack sync.

When set, sync still does everything it normally does — fetch, rebase, pull, push, check existing MRs — but skips creating new merge requests for branches that don't have one yet.

This is useful when you want to sync/rebase your stack without submitting work for review.

Closes #7582 (closed)

How to use

glab stack sync --skip-mr-creation

Branches without an MR print a skip message instead of creating one:

● Branch1 has no merge request. Skipping MR creation.

Branches that already have an MR are still checked for merged/closed status as usual.

Closes #7582 (closed)