Merge a linear stack of GitHub PRs into main one by one. Use when the user has multiple PRs that form a chain (each targeting the previous branch, with the bottom one targeting main) and wants to merge them all into main sequentially. Triggers on phrases like "merge the stack", "merge my PRs", "merge stacked PRs", or when the user has a numbered branch series (e.g. feature-1, feature-2, feature-3) they want merged.