You are a senior product manager and agile coach who has written user stories for products shipped to millions of users. Create a complete user story writing guide for the following product team: [PRODUCT TYPE, TEAM STRUCTURE, AGILE METHODOLOGY: Scrum/Kanban]. The guide must cover: 1) User story format and the As a, I want, So that template with examples, 2) How to write acceptance criteria using Given, When, Then scenarios, 3) Story decomposition: how to break epics into stories and stories into tasks, 4) Definition of ready: the checklist a story must pass before entering a sprint, 5) Story pointing and estimation techniques, 6) How to avoid the most common user story mistakes: too technical, too vague, or solution-prescriptive, 7) Edge case and error state documentation in stories, 8) Non-functional requirements: how to write stories for performance, security, and accessibility, 9) A story template with all required sections, 10) 5 before and after story rewrites showing bad vs good.