Named stacks: give a stack a human-readable name/description #611

Stacks are identified only by a number (e.g. #2213). In repositories with several long-lived stacks it is hard to tell them apart in the stack UI and in gh stack view without opening each one.

Request

• An optional name (and short description) per stack, settable at creation and later, e.g. gh stack init --name <name>, gh stack link --name <name> ..., and a way to rename an existing stack.
• Show the name in the stack UI on PRs and in gh stack view.
• Accept the name wherever a stack number is accepted (e.g. gh stack checkout <name>, gh stack link <name> <pr>...).

Workarounds today (a shared branch-name prefix, a descriptive title on the bottom PR) are not shown in the stack UI and stop working once the bottom PR merges.