gat-milestone

# 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` Do NOT fail when no system GDDs exist. Planning runs before design; the systems index's priorities and dependencies are sufficient input for slicing. Read: - `gat/overview/game.md` - `gat/overview/systems-index.md` - `gat/overview/art-direction.md` - all existing `gat/narrative/*.md` (used to inform milestone boundaries) - `.claude/docs/templates/plan/milestone.md` - `.claude/docs/templates/plan/m-brief.md` - `gat/milestone/milestone.md` if it already exists - existing `gat/milestone/m{N}-<name>/m{N}-brief.md` files if any milestones are already planned ## Phase 2: Hand Off To The Planner Spawn `gat-planner` agent with all read content plus: - instruction to write or update `gat/milestone/milestone.md` as an ordered set of milestone slices (`M01`, `M02`, ...) with a status column (`planned -> desi...