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 -> designing -> designed)
• instruction to create a directory gat/milestone/m{N}-<name>/ for each planned milestone
• instruction to author an m{N}-brief.md skeleton (template: .claude/docs/templates/plan/m-brief.md) for each milestone, listing every in-scope system with status Pending and the milestone status planned
• instruction to add an internal iteration plan to each brief using the template's repeated subsections;
• instruction NOT to write system GDDs, content-data docs, art docs, task lists, technical designs, prompt packs, or code
• planning focus from argument if provided

The planner should:

• choose a small set of meaningful milestones that can be handed off one stage at a time
• give each milestone a clear goal, player-facing outcome, and named system set
• define what is in scope and explicitly out of scope for each milestone
• use the systems index priorities + dependencies + narrative structure to slice; it does not need per-system rule detail
• include overview, narrative (scoped range), and content context needed by a downstream engineering workflow
• avoid technical architecture, file plans, coding tasks, and implementation sequencing
• set each new milestone's status to planned

Phase 3: Review

Summarize how many milestones were planned, which comes first, and which directories/briefs were created.

Use AskUserQuestion:

• Start designing the first milestone (Recommended) → Tell the user to run /gat-design <first-milestone> <system> or /gat-design <first-milestone> (continue)
• Stop here