Generates a pull request description by reading the git diff and the linked user story, mapping each changed file to an acceptance criterion so reviewers know why each change exists. Use when a developer says things like "write my PR description", "generate a PR", "help me describe this pull request", "create the PR body", "what should I put in my PR", or is about to open a pull request. Do not use for code review, story writing, or general git help unrelated to a pull request.
