Creates an implementation-ready plan for a feature, bug fix, refactor, or chore from a PRD, issue, document, or description using codebase evidence, first-principles reasoning, and conditional root-cause analysis, research, or spikes. Publishes issue-derived plans back to their source issue. Use when the user asks to "plan this feature", "plan this bug fix", "plan issue X", "create an implementation plan", "turn this PRD into a plan", investigate how a change should be built, link related plans, or invokes $prp-plan.