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>

bin/gstack-design-md.ts

@@ -53,8 +53,8 @@ export function main(argv = process.argv.slice(2)): number {
       const loaded = load(file);
       const { format, reason } = detectFormat(loaded?.doc ?? null);
       process.stdout.write(`${SENTINEL.DESIGN_MD_FORMAT}: ${format}\n`);
-      if (reason) process.stdout.write(`DESIGN_MD_REASON: ${reason}\n`);
-      process.stdout.write(`DESIGN_MD_MARKER: ${loaded?.doc.marker ?? 'none'}\n`);
+      if (reason) process.stdout.write(`${SENTINEL.DESIGN_MD_REASON}: ${reason}\n`);
+      process.stdout.write(`${SENTINEL.DESIGN_MD_MARKER}: ${loaded?.doc.marker ?? 'none'}\n`);
       return 0;
     }
     case 'convert': {
@@ -73,7 +73,7 @@ export function main(argv = process.argv.slice(2)): number {
       if (flags.has('--write')) {
         fs.writeFileSync(`${file}.legacy.bak`, loaded.text);
         writeAtomic(file, out);
-        process.stdout.write(`${SENTINEL.DESIGN_MD_FORMAT}: spec\nDESIGN_MD_WRITTEN: ${file}\nDESIGN_MD_BACKUP: ${file}.legacy.bak\n`);
+        process.stdout.write(`${SENTINEL.DESIGN_MD_FORMAT}: spec\n${SENTINEL.DESIGN_MD_WRITTEN}: ${file}\n${SENTINEL.DESIGN_MD_BACKUP}: ${file}.legacy.bak\n`);
       } else {
         process.stdout.write(out);
       }
@@ -97,7 +97,7 @@ export function main(argv = process.argv.slice(2)): number {
       const loaded = load(file);
       if (!loaded) { process.stdout.write(`${SENTINEL.DESIGN_MD_FORMAT}: missing\n`); return 1; }
       writeAtomic(file, renderDesignMd(setMarker(loaded.doc, choice)));
-      process.stdout.write(`DESIGN_MD_MARKER: ${choice}\n`);
+      process.stdout.write(`${SENTINEL.DESIGN_MD_MARKER}: ${choice}\n`);
       return 0;
     }
     default: