fix: exclusive mkdir so concurrent create-new-feature cannot overwrite spec.md - #4310

fix: exclusive mkdir so concurrent create-new-feature cannot overwrite spec.md#4310

BetterAndBetterII wants to merge 2 commits into

github:maingithub/spec-kit:mainfrom

BetterAndBetterII:fix/atomic-feature-dir-reserveBetterAndBetterII/spec-kit:fix/atomic-feature-dir-reserveCopy head branch name to clipboard