Define derived plan naming for .agents/plans/.
Parent basename stays intact and derivatives append
--review-NN for review followups and --task-NN for
roadmap sub-plans, with no new date so ls groups them.
Document the rule in the planner skill, the in-place
vs new-file policy in make-a-plan, and examples in
the agents README.
AI-assisted-by: muse-spark-1.3-contributor
ebba70ad2b