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-review/SKILL.md

@@ -453,6 +453,14 @@ You are a senior product designer AND a frontend engineer. Review live sites wit
 
 Look for `DESIGN.md`, `design-system.md`, or similar in the repo root. If found, read it — all design decisions must be calibrated against it. Deviations from the project's stated design system are higher severity. If not found, use universal design principles and offer to create one from the inferred system.
 
+**DESIGN.md format:**
+
+```bash
+bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts check DESIGN.md
+```
+
+`DESIGN_MD_FORMAT: spec`: the front matter is normative. Run `bun --no-env-file run $HOME/.claude/skills/gstack/bin/gstack-design-md.ts tokens DESIGN.md` and calibrate against the flat token map: a value present there is never a finding, and a finding that departs from a token names the token. `legacy` or `unknown`: read the file as prose. The `DESIGN_MD_MARKER` line is the user's persisted format choice; respect it and never offer a conversion here (that is /design-consultation's question). `missing`: universal principles.
+
 **Check for clean working tree:**
 
 ```bash
@@ -1851,4 +1859,4 @@ already knows. A good test: would this insight save time in a future session? If
 14. **Revert on regression.** If a fix makes things worse, `git revert HEAD` immediately.
 15. **Self-regulate.** Follow the design-fix risk heuristic. When in doubt, stop and ask.
 16. **CSS-first.** Prefer CSS/styling changes over structural component changes. CSS-only changes are safer and more reversible.
-17. **DESIGN.md export.** You MAY write a DESIGN.md file if the user accepts the offer from Phase 2.
+17. **DESIGN.md export.** You MAY write a DESIGN.md file if the user accepts the offer from Phase 2. Write it in the open DESIGN.md format (front matter tokens plus the canonical sections, the Phase 6 template in /design-consultation); an existing file keeps its persisted format choice, and this skill never offers a conversion.

design-review/SKILL.md.tmpl

@@ -52,6 +52,8 @@ You are a senior product designer AND a frontend engineer. Review live sites wit
 
 Look for `DESIGN.md`, `design-system.md`, or similar in the repo root. If found, read it — all design decisions must be calibrated against it. Deviations from the project's stated design system are higher severity. If not found, use universal design principles and offer to create one from the inferred system.
 
+{{DESIGN_MD_CHECK:calibrate}}
+
 **Check for clean working tree:**
 
 ```bash
@@ -327,4 +329,4 @@ If the repo has a `TODOS.md`:
 14. **Revert on regression.** If a fix makes things worse, `git revert HEAD` immediately.
 15. **Self-regulate.** Follow the design-fix risk heuristic. When in doubt, stop and ask.
 16. **CSS-first.** Prefer CSS/styling changes over structural component changes. CSS-only changes are safer and more reversible.
-17. **DESIGN.md export.** You MAY write a DESIGN.md file if the user accepts the offer from Phase 2.
+17. **DESIGN.md export.** You MAY write a DESIGN.md file if the user accepts the offer from Phase 2. Write it in the open DESIGN.md format (front matter tokens plus the canonical sections, the Phase 6 template in /design-consultation); an existing file keeps its persisted format choice, and this skill never offers a conversion.