Update Implementation Plan skill: what it does and how to install it

What it does. Updates an existing Markdown implementation plan with changed requirements for features, refactoring, package upgrades, design, architecture or infrastructure. It organises the plan into phased tasks with identifiers, dependencies, completion criteria, affected files and tests, using a required front matter and section structure.

When to use it. Use it when an existing implementation plan must be revised for new or updated requirements and made executable by agents or people. It also defines status, naming and validation rules for the resulting plan.