Name or alias stacks in the CLI and PR UI

[rileymeye1]

On our team every stack maps to a Linear ticket (SAL-NNNN), but stacks only show up as numbers in gh stack checkout and as unlabeled chains in the PR UI. When several stacks are open at once, I can't tell which is which without opening the PRs.

A name would fix this for us, set at gh stack init --name "SAL-1234 checkout flow" and editable later. We'd want it shown in gh stack view, accepted by gh stack checkout , and displayed as a label on the stack in the PR page and PR list. Grouping in list views (#474) would pair well with it.

This would be well suited to work alongside #261 bringing the wants into 1 total feature.