Milestone

This skill plans milestone slices BEFORE per-system design and creates:

• gat/milestone/milestone.md — the ordered milestone roadmap with status
• gat/milestone/m{N}-<name>/ — per-milestone directory skeletons
• gat/milestone/m{N}-<name>/m{N}-brief.md — milestone brief skeletons (all systems Pending, status planned)

It does NOT require any system GDD to exist. It does not create task lists, technical designs, prompt packs, or implementation files. Those belong to the downstream engineering workflow.

Phase 1: Validate Inputs

Fail if any of these are missing:

• gat/overview/game.md
• gat/overview/systems-index.md
• gat/overview/art-direction.md