gat-design

# Design This skill designs a single system within a specific milestone, after `/gat-brainstorm` (overview) and `/gat-milestone` (milestone plan) are done. For story, worldbuilding, characters, quests, dialogue, or authored narrative content, delegate to `/gat-story`. For the initial concept interview, delegate to `/gat-brainstorm`. The `milestone` argument is **mandatory**. If it is omitted, the skill stops and asks the user to specify a milestone — no inference, default, or fallback. ## Phase 1: Resolve Mode Check existing files: - If `gat/overview/game.md` or `gat/overview/systems-index.md` missing → hand off: tell the user to run `/gat-brainstorm [hint]` first. Stop. - If `gat/overview/art-direction.md` missing → hand off: tell the user to run `/gat-brainstorm [hint]` to establish global art direction. Stop. - If `gat/milestone/milestone.md` missing → hand off: tell the user to run `/gat-milestone` first. Stop. - If no `milestone` argument provided → STOP. Tell the user a milestone is required and show the milestones listed in `gat/milestone/milestone.md`. Do NOT infer or default one. Do NOT write any files. - If the `milestone` argument does not match any milestone directory under `gat/milestone/` (i.e., no `m{N}-<name>/m{N}-brief.md` exists) → STOP. Tell the user the milestone was not found and list the valid milestones. Then resolve sub-mode: - If a `system` (or hint) argument is also provided → Mode: `system` (design that one system in the milestone) - If only ...