Turn a prompt, bug report, issue ID, or ticket URL into a repository-grounded implementation plan for human approval. Use when the user asks to plan a PR, prepare a ticket for implementation, work out how to build a feature, or set up the plan-pr workflow. Reads tracker context and repo rules, routes to a fixed set of design and TDD skills, and defines observable acceptance criteria. After approval the same agent implements, then invokes ship-pr-dev. For shipping an existing branch or fixing PR CI without a planning request, use ship-pr-dev directly.