A milestone is a named state where a set of closure conditions are simultaneously met. It represents a meaningful intermediate closure — a state where enough gaps have been resolved that a new capability exists.
Milestones are declared as milestone: values in plan frontmatter.
Plans that share a milestone value form a coherent group whose
collective completion constitutes the named state.
Milestones are not deadlines. They are descriptions of closure states, ordered by dependency, not by calendar.