feat(design): write/read DESIGN.md in the open spec; persisted format choice

gstack's design skills now write DESIGN.md in the open DESIGN.md format and
read tokens from it. {{DESIGN_MD_CHECK}} renders the format check through
bin/gstack-design-md.ts: design-consultation's Phase 0 settles the format once
(spec → update tokens in the front matter; legacy without a marker → one
AskUserQuestion: convert with a .legacy.bak, keep the legacy file, or start
fresh; the answer is written into the file as the format marker so no skill
asks again; a marker already present is obeyed silently; unknown → prose;
missing → Phase 6 writes one). Phase 6's template is the spec form: YAML front
matter with name, description, and exactly the five token groups (colors,
typography.display/body/label/mono with fontFeature: tnum on mono, rounded,
spacing, components with {path} references), then Overview (Creative North
Star, product context, mode per surface, references, key characteristics),
Colors (opening with the Restrained / Committed / Full palette / Drenched
strategy), Typography, Layout, Elevation & Depth, Shapes, Components, Do's and
Don'ts, plus gstack's Motion and Decisions Log as extras; the template ends
with a check that the file parses as `spec`.

design-review runs the `:calibrate` form in Setup: a spec file's flat tokens
are the calibration source (a value present in the tokens is never a finding),
the marker is respected, and conversion is never offered there; its DESIGN.md
export writes the spec form. design-html's token extraction writes the spec
form and respects an existing choice. review/design-checklist.md category 5 and
ship's review-lite step 1 name `gstack-design-md tokens` as the calibration
source; plan-design-review Pass 5 cites tokens by path when front matter
exists.

The contract owns the bin's DESIGN_MD_MARKER / REASON / WRITTEN / BACKUP
lines; the contract test's pending list closes. Carve guard: design-
consultation skeleton 66,500 → 67,500 (measured 67,014; +1,508 B against the
1.5 KB cap). Codex and Factory ship goldens refreshed (review-lite step 1).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

design-consultation/SKILL.md

@@ -466,7 +466,19 @@ You are a senior product designer with strong opinions about typography, color,
 ls DESIGN.md design-system.md 2>/dev/null || echo "NO_DESIGN_FILE"
 ```
 
-- If a DESIGN.md exists: Read it. Ask the user: "You already have a design system. Want to **update** it, **start fresh**, or **cancel**?"
+- If a DESIGN.md exists: Read it. Ask the user: "You already have a design system. Want to **update** it, **start fresh**, or **cancel**?" Then settle its format once:
+
+**DESIGN.md format** (the open format; Phase 6 has the template):
+
+```bash
+bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts check DESIGN.md
+```
+
+- `DESIGN_MD_FORMAT: spec` → already the open format; `bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts tokens DESIGN.md` prints the flat token map. Update tokens in the front matter, rationale in the sections.
+- `legacy` with `DESIGN_MD_MARKER: none` → ask once (AskUserQuestion): **A) Convert** (recommended; `bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts convert --write` keeps a `.legacy.bak` and every section) **B) Keep legacy** (`bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts mark legacy-keep`; read as prose from now on) **C) Start fresh**. The answer lives in the file, so no skill asks again; a marker already present is obeyed silently.
+- `unknown` → read as prose, say why once (`DESIGN_MD_REASON`); `DESIGN_MD_CONVERT_REFUSED` means both formats are mixed: leave it, tell the user.
+- `missing` → Phase 6 writes one. Exit 3 (`DESIGN_MD_INTERNAL_ERROR`) is a gstack bug: report it, do not retry.
+
 - If no DESIGN.md: continue.
 
 **Gather product context from the codebase:**