resolve-pr-comments-stack

Resolve unresolved PR review comments across an entire Graphite (gt) stack of many PRs, bottom-up, in one working directory. Use when asked to "go through this stack and resolve comments", "clean up review comments across the whole stack", or given a list/range of PR numbers that form (or partially form) a gt stack. Distinct from the single-PR resolve-pr-comments skill, which this one uses as its per-branch inner…