create-milestones

Create, advance, and complete milestones for a project whose foundation work is done. Drives the 10-phase TDD methodology (Define Contract, Write Tests RED, Create Fixtures, Run Tests RED, Update Interfaces, Implement Core, Update Wrappers, Run Tests GREEN, Integrate, Quality/Docs). Authors milestone + impl-log pairs via `docs new` and atomically archives them via `docs archive --cascade` on completion. Triggers on "create a milestone", "start M1", "next milestone", "begin implementation", "advance the project". Use after `project-foundation` has set up the docs tree.